@article{garcia_sakari_roederer_evans_silva_mateo_song_kremin_bailey iii_walker_2024, title={Abundances of Neutron-capture Elements in 62 Stars in the Globular Cluster Messier 15}, volume={967}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ad380b}, DOI={10.3847/1538-4357/ad380b}, abstractNote={Abstract M15 is a globular cluster with a known spread in neutron-capture elements. This paper presents abundances of neutron-capture elements for 62 stars in M15. Spectra were obtained with the Michigan/Magellan Fiber System spectrograph, covering a wavelength range from ∼4430 to 4630 Å. Spectral lines from Fe i , Fe ii , Sr i , Zr ii , Ba ii , La ii , Ce ii , Nd ii , Sm ii , Eu ii , and Dy ii were measured, enabling classifications and neutron-capture abundance patterns for the stars. Of the 62 targets, 44 are found to be highly Eu-enhanced r -II stars, another 17 are moderately Eu-enhanced r -I stars, and one star is found to have an s -process signature. The neutron-capture patterns indicate that the majority of the stars are consistent with enrichment by the r -process. The 62 target stars are found to show significant star-to-star spreads in Sr, Zr, Ba, La, Ce, Nd, Sm, Eu, and Dy, but no significant spread in Fe. The neutron-capture abundances are further found to have slight correlations with sodium abundances from the literature, unlike what has been previously found; follow-up studies are needed to verify this result. The findings in this paper suggest that the Eu-enhanced stars in M15 were enhanced by the same process, that the nucleosynthetic source of this Eu pollution was the r -process, and that the r -process source occurred as the first generation of cluster stars was forming.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Garcia, Jonathan Cabrera and Sakari, Charli M. and Roederer, Ian U. and Evans, Donavon W. and Silva, Pedro and Mateo, Mario and Song, Ying-Yi and Kremin, Anthony and Bailey III, John I. and Walker, Matthew G.}, year={2024}, month={Jun} }
 @article{den hartog_voith_roederer_2024, title={Atomic Transition Probabilities for Ultraviolet and Optical Lines of Tm ii}, volume={274}, ISSN={["1538-4365"]}, url={https://doi.org/10.3847/1538-4365/ad614f}, DOI={10.3847/1538-4365/ad614f}, abstractNote={Abstract We report new branching fraction measurements for 224 ultraviolet and optical transitions of Tm ii . These transitions range in wavelength (wavenumber) from 2350 to 6417 Å (42,532–15,579 cm −1 ) and originate in 13 odd-parity and 24 even-parity upper levels. Thirty-five of the 37 levels, accounting for 213 of the 224 transitions, are studied for the first time. Branching fractions are determined for two levels studied previously for comparison to earlier results. The levels studied for the first time are high lying, ranging in energy from 35,753 to 54,989 cm −1 . The branching fractions are determined from emission spectra from two different high-resolution spectrometers. These are combined with radiative lifetimes reported in an earlier study to produce a set of transition probabilities and log( gf ) values with accuracy ranging from 5% to 30%. Comparison is made to experimental and theoretical transition probabilities from the literature where such data exist. These new log( gf ) values are used to derive an abundance from one previously unused Tm ii line in the UV spectrum of the r -process-enhanced metal-poor star HD 222925, and this abundance is consistent with previous determinations based on other Tm ii lines.}, number={1}, journal={ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES}, author={Den Hartog, E. A. and Voith, G. T. and Roederer, I. U.}, year={2024}, month={Sep} }
 @article{kowkabany_ezzeddine_charbonnel_roederer_wang_li_hackshaw_beers_frebel_hansen_et al._2024, title={Discovery of a Metal-poor Red Giant Star with the Highest Ultralithium Enhancement}, volume={973}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ad6004}, DOI={10.3847/1538-4357/ad6004}, abstractNote={Abstract We present the discovery of 2MASS J05241392−0336543 (hereafter J0524−0336), a very metal-poor ([Fe/H] = −2.43 ± 0.16), highly r -process-enhanced ([Eu/Fe] = +1.34 ± 0.10) Milky Way halo field red giant star, with an ultrahigh Li abundance of A (Li, 3D, NLTE) = 6.15 ± 0.25 and [Li/Fe] = +7.64 ± 0.25, respectively. This makes J0524−0336 the most lithium-enhanced giant star discovered to date. We present a detailed analysis of the star’s atmospheric stellar parameters and chemical abundance determinations. Additionally, we detect indications of infrared excess, as well as observe variable emission in the wings of the H α absorption line across multiple epochs, indicative of a potential enhanced mass-loss event with possible outflows. Our analysis reveals that J0524−0336 lies either between the bump and the tip of the red giant branch (RGB), or on the early asymptotic giant branch (e-AGB). We investigate the possible sources of lithium enrichment in J0524−0336, including both internal and external sources. Based on current models and on the observational evidence we have collected, our study shows that J0524−0336 may be undergoing the so-called lithium flash that is expected to occur in low-mass stars when they reach the RGB bump and/or the e-AGB.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Kowkabany, Jeremy and Ezzeddine, Rana and Charbonnel, Corinne and Roederer, Ian U. and Wang, Ella, Xi and Li, Yangyang and Hackshaw, Zoe and Beers, Timothy C. and Frebel, Anna and Hansen, Terese T. and et al.}, year={2024}, month={Oct} }
 @article{amaducci_colonna_cosentino_cristallo_finocchiaro_krticka_massimi_mastromarco_mazzone_maugeri_et al._2024, title={Measurement of the 140Ceðn;γþ Cross Section at n_TOF and Its Astrophysical Implications for the Chemical Evolution of the Universe}, volume={132}, ISSN={["1079-7114"]}, DOI={10.1103/PhysRevLett.132.122701}, abstractNote={<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mmultiscripts><a:mrow><a:mi>Ce</a:mi></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>140</a:mn></a:mrow></a:mmultiscripts><a:mo stretchy="false">(</a:mo><a:mi mathvariant="normal">n</a:mi><a:mo>,</a:mo><a:mi>γ</a:mi><a:mo stretchy="false">)</a:mo></a:mrow></a:math> is a key reaction for slow neutron-capture (<f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mi>s</f:mi></f:math>-process) nucleosynthesis due to being a bottleneck in the reaction flow. For this reason, it was measured with high accuracy (uncertainty <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mo>≈</h:mo><h:mn>5</h:mn><h:mo>%</h:mo></h:math>) at the n_TOF facility, with an unprecedented combination of a high purity sample and low neutron-sensitivity detectors. The measured Maxwellian averaged cross section is up to 40% higher than previously accepted values. Stellar model calculations indicate a reduction around 20% of the <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mi>s</j:mi></j:math>-process contribution to the Galactic cerium abundance and smaller sizeable differences for most of the heavier elements. No variations are found in the nucleosynthesis from massive stars. Published by the American Physical Society 2024}, number={12}, journal={PHYSICAL REVIEW LETTERS}, author={Amaducci, S. and Colonna, N. and Cosentino, L. and Cristallo, S. and Finocchiaro, P. and Krticka, M. and Massimi, C. and Mastromarco, M. and Mazzone, A. and Maugeri, E. A. and et al.}, year={2024}, month={Mar} }
 @article{roederer_beers_hattori_placco_hansen_ezzeddine_frebel_holmbeck_sakari_2024, title={The <i>R</i>-Process Alliance: 2MASS J22132050-5137385, the Star with the Highest-known <i>r</i>-process Enhancement at [Eu/Fe] =+2.45}, volume={971}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ad57bf}, DOI={10.3847/1538-4357/ad57bf}, abstractNote={Abstract We present stellar parameters and chemical abundances of 47 elements detected in the bright ( V = 11.63) very metal-poor ([Fe/H] = −2.20 ± 0.12) star 2MASS J22132050−5137385. We observed this star using the Magellan Inamori Kyocera Echelle spectrograph as part of ongoing work by the R -Process Alliance. The spectrum of 2MASS J22132050−5137385 exhibits unusually strong lines of elements heavier than the iron group, and our analysis reveals that these elements were produced by rapid neutron-capture ( r -process) nucleosynthesis. We derive a europium enhancement, [Eu/Fe] = +2.45 ± 0.08, that is higher than any other r -process-enhanced star known at present. This star is only the eighth r -process-enhanced star where both thorium and uranium are detected, and we calculate the age of the r -process material, 13.6 ± 2.6 Gyr, from the radioactive decay of these isotopes. This star contains relatively large enhancements of elements that may be produced as transuranic fission fragments, and we propose a new method using this characteristic to assess the r -process yields and gas dilution in samples of r -process-enhanced stars. Assuming a canonical baryonic minihalo mass of 10 6 M ⊙ and a 1% metal retention rate, this star formed in a cloud of only ∼600 M ⊙ . We conclude that 2MASS J22132050−5137385 exhibits a high level of r -process enhancement because it formed in an environment where the r -process material was less diluted than average.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Roederer, Ian U. and Beers, Timothy C. and Hattori, Kohei and Placco, Vinicius M. and Hansen, Terese T. and Ezzeddine, Rana and Frebel, Anna and Holmbeck, Erika M. and Sakari, Charli M.}, year={2024}, month={Aug} }
 @article{xylakis-dornbusch_hansen_beers_christlieb_ezzeddine_frebel_holmbeck_placco_roederer_sakari_et al._2024, title={The <i>R</i>-Process Alliance: Analysis of limited-<i>r</i> stars}, volume={688}, ISSN={["1432-0746"]}, DOI={10.1051/0004-6361/202449376}, abstractNote={Context. In recent years, the R -Process Alliance (RPA) has conducted a successful search for stars that are enhanced in elements produced by the rapid neutron-capture ( r -)process. In particular, the RPA has uncovered a number of stars that are strongly enriched in light r -process elements, such as Sr, Y, and Zr. These so-called limited- r stars were investigated to explore the astrophysical production site(s) of these elements. Aims. We investigate the possible formation sites for light neutron-capture elements by deriving detailed abundances for neutron-capture elements from high-resolution spectra with a high signal-to-noise ratio of three limited- r stars. Methods. We conducted a kinematic analysis and a 1D local thermodynamic equilibrium spectroscopic abundance analysis of three stars. Furthermore, we calculated the lanthanide mass fraction ( X La ) of our stars and of limited- r stars from the literature. Results. We found that the abundance pattern of neutron-capture elements of limited- r stars behaves differently depending on their [Ba/Eu] ratios, and we suggest that this should be taken into account in future investigations of their abundances. Furthermore, we found that the X La of limited- r stars is lower than that of the kilonova AT2017gfo. The latter seems to be in the transition zone between limited- rX La and that of r -I and r -II stars. Finally, we found that unlike r -I and r -II stars, the current sample of limited- r stars is largely born in the Galaxy and is not accreted.}, journal={ASTRONOMY & ASTROPHYSICS}, author={Xylakis-Dornbusch, T. and Hansen, T. T. and Beers, T. C. and Christlieb, N. and Ezzeddine, R. and Frebel, A. and Holmbeck, E. and Placco, V. M. and Roederer, I. U. and Sakari, C. M. and et al.}, year={2024}, month={Aug} }
 @article{bandyopadhyay_ezzeddine_prieto_aria_shah_beers_frebel_hansen_holmbeck_placco_et al._2024, title={The <i>R</i>-process Alliance: Fifth Data Release from the Search for <i>R</i>-process-enhanced Metal-poor Stars in the Galactic Halo with the GTC}, volume={274}, ISSN={["1538-4365"]}, DOI={10.3847/1538-4365/ad6f0f}, abstractNote={Abstract Understanding the abundance pattern of metal-poor stars and the production of heavy elements through various nucleosynthesis processes offers crucial insights into the chemical evolution of the Milky Way, revealing primary sites and major sources of rapid neutron-capture process ( r -process) material in the Universe. In this fifth data release from the R -Process Alliance (RPA), we present the detailed chemical abundances of 41 faint (down to V = 15.8) and extremely metal-poor (down to [Fe/H] = −3.3) halo stars selected from the RPA. We obtained high-resolution spectra for these objects with the HORuS spectrograph on the Gran Telescopio Canarias. We measure the abundances of light, α , Fe-peak, and neutron-capture elements. We report the discovery of five carbon-enhanced metal-poor, one limited- r , three r -I, and four r -II stars, and six Mg-poor stars. We also identify one star of a possible globular cluster origin at an extremely low metallicity at [Fe/H] = −3.0. This adds to the growing evidence of a lower-limit metallicity floor for globular cluster abundances. We use the abundances of Fe-peak elements and the α -elements to investigate the contributions from different nucleosynthesis channels in the progenitor supernovae. We find the distribution of [Mg/Eu] as a function of [Fe/H] to have different enrichment levels, indicating different possible pathways and sites of their production. We also reveal differences in the trends of the neutron-capture element abundances of Sr, Ba, and Eu of various r -I and r -II stars from the RPA data releases, which provide constraints on their nucleosynthesis sites and subsequent evolution.}, number={2}, journal={ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES}, author={Bandyopadhyay, Avrajit and Ezzeddine, Rana and Prieto, Carlos Allende and Aria, Nima and Shah, Shivani P. and Beers, Timothy C. and Frebel, Anna and Hansen, Terese T. and Holmbeck, Erika M. and Placco, Vinicius M. and et al.}, year={2024}, month={Oct} }
 @article{shah_ezzeddine_roederer_hansen_placco_beers_frebel_ji_holmbeck_marshall_et al._2024, title={The R-Process Alliance: detailed chemical composition of an r-process enhanced star with UV and optical spectroscopy}, volume={529}, ISSN={["1365-2966"]}, DOI={10.1093/mnras/stae255}, abstractNote={ABSTRACT
               We present a detailed chemical-abundance analysis of a highly r-process-enhanced (RPE) star, 2MASS J00512646-1053170, using high-resolution spectroscopic observations with Hubble Space Telescope/STIS in the UV and Magellan/MIKE in the optical. We determined abundances for 41 elements in total, including 23 r-process elements and rarely probed species such as Al ii, Ge i, Mo ii, Cd i, Os ii, Pt i, and Au i. We find that [Ge/Fe] = +0.10, which is an unusually high Ge enhancement for such a metal-poor star and indicates contribution from a production mechanism decoupled from that of Fe. We also find that this star has the highest Cd abundance observed for a metal-poor star to date. We find that the dispersion in the Cd abundances of metal-poor stars can be explained by the correlation of Cd i abundances with the stellar parameters of the stars, indicating the presence of NLTE effects. We also report that this star is now only the sixth star with Au abundance determined. This result, along with abundances of Pt and Os, uphold the case for the extension of the universal r-process pattern to the third r-process peak and to Au. This study adds to the sparse but growing number of RPE stars with extensive chemical-abundance inventories and highlights the need for not only more abundance determinations of these rarely probed species, but also advances in theoretical NLTE and astrophysical studies to reliably understand the origin of r-process elements.}, number={3}, journal={MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY}, author={Shah, Shivani P. and Ezzeddine, Rana and Roederer, Ian U. and Hansen, Terese T. and Placco, Vinicius M. and Beers, Timothy C. and Frebel, Anna and Ji, Alexander P. and Holmbeck, Erika M. and Marshall, Jennifer and et al.}, year={2024}, month={Mar}, pages={1917–1940} }
 @article{roederer_alvarado-gomez_allende prieto_adibekyan_aguado_amado_amazo-gomez_baratella_barnes_bensby_et al._2024, title={The discovery space of ELT-ANDES. Stars and stellar populations}, volume={57}, ISSN={["1572-9508"]}, DOI={10.1007/s10686-024-09938-8}, number={2}, journal={EXPERIMENTAL ASTRONOMY}, author={Roederer, Ian U. and Alvarado-Gomez, Julian D. and Allende Prieto, Carlos and Adibekyan, Vardan and Aguado, David S. and Amado, Pedro J. and Amazo-Gomez, Eliana M. and Baratella, Martina and Barnes, Sydney A. and Bensby, Thomas and et al.}, year={2024}, month={Apr} }
 @article{roederer_pace_placco_caldwell_koposov_mateo_olszewski_walker_2023, title={Abundance Analysis of Stars at Large Radius in the Sextans Dwarf Spheroidal Galaxy*}, volume={954}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/ace3c1}, DOI={10.3847/1538-4357/ace3c1}, abstractNote={Abstract
               We present the stellar parameters and chemical abundances of 30 elements for five stars located at large radii (3.5–10.7 times the half-light radius) in the Sextans dwarf spheroidal galaxy. We selected these stars using proper motions, radial velocities, and metallicities, and we confirm them as metal-poor members of Sextans with −3.34 ≤ [Fe/H] ≤ −2.64 using high-resolution optical spectra collected with the Magellan Inamori Kyocera Echelle spectrograph. Four of the five stars exhibit normal abundances of C (−0.34 ≤ [C/Fe] ≤ + 0.36), mild enhancement of the α elements Mg, Si, Ca, and Ti ([α/Fe] = +0.12 ± 0.03), and unremarkable abundances of Na, Al, K, Sc, V, Cr, Mn, Co, Ni, and Zn. We identify three chemical signatures previously unknown among stars in Sextans. One star exhibits large overabundances ([X/Fe] > +1.2) of C, N, O, Na, Mg, Si, and K, and large deficiencies of heavy elements ([Sr/Fe] = −2.37 ± 0.25, [Ba/Fe] = −1.45 ± 0.20, [Eu/Fe] < + 0.05), establishing it as a member of the class of carbon-enhanced metal-poor stars with no enhancement of neutron-capture elements. Three stars exhibit moderate enhancements of Eu (+0.17 ≤ [Eu/Fe] ≤ + 0.70), and the abundance ratios among 12 neutron-capture elements are indicative of r-process nucleosynthesis. Another star is highly enhanced in Sr relative to heavier elements ([Sr/Ba] = +1.21 ± 0.25). These chemical signatures can all be attributed to massive, low-metallicity stars or their end states. Our results, the first for stars at large radius in Sextans, demonstrate that these stars were formed in chemically inhomogeneous regions, such as those found in ultra-faint dwarf galaxies.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Pace, Andrew B. and Placco, Vinicius M. and Caldwell, Nelson and Koposov, Sergey E. and Mateo, Mario and Olszewski, Edward W. and Walker, Matthew G.}, year={2023}, month={Aug}, pages={55} }
 @article{den hartog_lawler_sneden_roederer_cowan_2023, title={Atomic Transition Probabilities for Transitions of Si i and Si ii and the Silicon Abundances of Several Very Metal-poor Stars
                  <sup>∗
               
                }, volume={265}, ISSN={0067-0049 1538-4365}, url={http://dx.doi.org/10.3847/1538-4365/acb642}, DOI={10.3847/1538-4365/acb642}, abstractNote={Abstract
               We report new measurements of branching fractions for 20 UV and blue lines in the spectrum of neutral silicon (Si i) originating in the 3s
                  23p4s
                  3Po
                  1,2,1Po
                  1, and 3s3p
                  3
                  1Do
                  1,2 upper levels. Transitions studied include both strong, nearly pure LS multiplets as well as very weak spin-forbidden transitions connected to these upper levels. We also report a new branching fraction measurement of the 4P1/2–2Po
                  1/2,3/2 intercombination lines in the spectrum of singly ionized silicon (Si ii). The weak spin-forbidden lines of Si i and Si ii provide a stringent test on recent theoretical calculations, to which we make comparison. The branching fractions from this study are combined with previously reported radiative lifetimes to yield transition probabilities and log(gf) values for these lines. We apply these new measurements to abundance determinations in five metal-poor stars.}, number={2}, journal={The Astrophysical Journal Supplement Series}, publisher={American Astronomical Society}, author={Den Hartog, E. A. and Lawler, J. E. and Sneden, C. and Roederer, I. U. and Cowan, J. J.}, year={2023}, month={Mar}, pages={42} }
 @article{roederer_vassh_holmbeck_mumpower_surman_cowan_beers_ezzeddine_frebel_hansen_et al._2023, title={Element abundance patterns in stars indicate fission of nuclei heavier than uranium}, volume={382}, ISSN={["1095-9203"]}, url={https://doi.org/10.1126/science.adf1341}, DOI={10.1126/science.adf1341}, abstractNote={
            The heaviest chemical elements are naturally produced by the rapid neutron-capture process (
            r
            -process) during neutron star mergers or supernovae. The
            r
            -process production of elements heavier than uranium (transuranic nuclei) is poorly understood and inaccessible to experiments so must be extrapolated by using nucleosynthesis models. We examined element abundances in a sample of stars that are enhanced in
            r
            -process elements. The abundances of elements ruthenium, rhodium, palladium, and silver (atomic numbers
            Z
            = 44 to 47; mass numbers
            A
            = 99 to 110) correlate with those of heavier elements (63 ≤
            Z
            ≤ 78,
            A
            > 150). There is no correlation for neighboring elements (34 ≤
            Z
            ≤ 42 and 48 ≤
            Z
            ≤ 62). We interpret this as evidence that fission fragments of transuranic nuclei contribute to the abundances. Our results indicate that neutron-rich nuclei with mass numbers >260 are produced in
            r
            -process events.
          }, number={6675}, journal={SCIENCE}, author={Roederer, Ian U. and Vassh, Nicole and Holmbeck, Erika M. and Mumpower, Matthew R. and Surman, Rebecca and Cowan, John J. and Beers, Timothy C. and Ezzeddine, Rana and Frebel, Anna and Hansen, Terese T. and et al.}, year={2023}, month={Dec} }
 @article{roederer_vassh_holmbeck_mumpower_surman_cowan_beers_ezzeddine_frebel_hansen_et al._2023, title={Evidence for transuranic fission fragments in stars}, volume={55}, number={2}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U. and Vassh, N. and Holmbeck, N. and Mumpower, E. and Surman, M. and Cowan, R. and Beers, J. and Ezzeddine, T. and Frebel, R. and Hansen, A. and et al.}, year={2023} }
 @article{hattori_okuno_roederer_2023, title={Finding r-II Sibling Stars in the Milky Way with the Greedy Optimistic Clustering Algorithm}, volume={946}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/acb93b}, DOI={10.3847/1538-4357/acb93b}, abstractNote={Abstract
               
                  R-process enhanced stars with [Eu/Fe] ≥ +0.7 (so-called r-II stars) are believed to have formed in an extremely neutron-rich environment in which a rare astrophysical event (e.g., a neutron-star merger) occurred. This scenario is supported by the existence of an ultra-faint dwarf galaxy, Reticulum II, where most of the stars are highly enhanced in r-process elements. In this scenario, some small fraction of dwarf galaxies around the Milky Way were r enhanced. When each r-enhanced dwarf galaxy accreted to the Milky Way, it deposited many r-II stars in the Galactic halo with similar orbital actions. To search for the remnants of the r-enhanced systems, we analyzed the distribution of the orbital actions of N = 161 r-II stars in the solar neighborhood by using Gaia EDR3 data. Since the observational uncertainty is not negligible, we applied a newly developed greedy optimistic clustering method to the orbital actions of our sample stars. We found six clusters of r-II stars that have similar orbits and chemistry, one of which is a new discovery. Given the apparent phase-mixed orbits of the member stars, we interpret that these clusters are good candidates for remnants of completely disrupted r-enhanced dwarf galaxies that merged with the ancient Milky Way.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Hattori, Kohei and Okuno, Akifumi and Roederer, Ian U.}, year={2023}, month={Mar}, pages={48} }
 @article{holmbeck_surman_roederer_mclaughlin_frebel_2023, title={HD 222925: A New Opportunity to Explore the Astrophysical and Nuclear Conditions of r-process Sites}, volume={951}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/acccf3}, DOI={10.3847/1538-4357/acccf3}, abstractNote={Abstract
               With the most trans-iron elements detected of any star outside the solar system, HD 222925 represents the most complete chemical inventory among metal-poor stars enhanced with elements made by the rapid neutron capture (“r”) process. As such, HD 222925 may be a new “template” for the observational r-process, where before the (much higher-metallicity) solar r-process residuals were used. In this work, we test under which conditions a single site accounts for the entire elemental r-process abundance pattern of HD 222925. We found that several of our tests—with the single exception of the black hole–neutron star merger case—challenge the single-site assumption by producing an ejecta distribution that is highly constrained, in disagreement with simulation predictions. However, we found that ejecta distributions that are more in line with simulations can be obtained under the condition that the nuclear data near the second r-process peak are changed. Therefore, for HD 222925 to be a canonical r-process template likely as a product of a single astrophysical source, the nuclear data need to be reevaluated. The new elemental abundance pattern of HD 222925—including the abundances obtained from space-based, ultraviolet (UV) data—call for a deeper understanding of both astrophysical r-process sites and nuclear data. Similar UV observations of additional r-process–enhanced stars will be required to determine whether the elemental abundance pattern of HD 222925 is indeed a canonical template (or an outlier) for the r-process at low metallicity.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Holmbeck, Erika M. and Surman, Rebecca and Roederer, Ian U. and McLaughlin, G. C. and Frebel, Anna}, year={2023}, month={Jun}, pages={30} }
 @article{sneden_boesgaard_cowan_roederer_den hartog_lawler_2023, title={Iron-peak Element Abundances in Warm Very Metal-poor Stars}, volume={953}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/accd62}, DOI={10.3847/1538-4357/accd62}, abstractNote={Abstract
               We have derived new detailed abundances of Mg, Ca, and the Fe-group elements Sc through Zn (Z = 21−30) for 37 main-sequence turnoff very metal-poor stars ([Fe/H] ≲−2.1). We analyzed Keck HIRES optical and near-UV high signal-to-noise spectra originally gathered for a Be abundance survey. Using typically ∼400 Fe-group lines with accurate laboratory transition probabilities for each star, we have determined accurate LTE metallicities and abundance ratios for neutral and ionized species of the 10 Fe-group elements as well as α elements Mg and Ca. We find good neutral/ion abundance agreement for the six elements that have detectable transitions of both species in our stars in the 3100–5800 Å range. Earlier reports of correlated Sc−Ti−V relative overabundances are confirmed, and appear to slowly increase with decreasing metallicity. To this element trio we add Zn; it also appears to be increasingly overabundant in the lowest-metallicity regimes. Co appears to mimic the behavior of Zn, but issues surrounding its abundance reliability cloud its interpretation.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Sneden, Christopher and Boesgaard, Ann Merchant and Cowan, John J. and Roederer, Ian U. and Den Hartog, Elizabeth A. and Lawler, James E.}, year={2023}, month={Jul}, pages={31} }
 @article{walker_caldwell_mateo_olszewski_pace_bailey_koposov_roederer_2023, title={Magellan/M2FS and MMT/Hectochelle Spectroscopy of Dwarf Galaxies and Faint Star Clusters within the Galactic Halo*}, volume={268}, ISSN={0067-0049 1538-4365}, url={http://dx.doi.org/10.3847/1538-4365/acdd79}, DOI={10.3847/1538-4365/acdd79}, abstractNote={Abstract
               We present spectroscopic data for 16,369 stellar targets within and/or toward 38 dwarf spheroidal galaxies and faint star clusters within the Milky Way halo environment. All spectra come from observations with the multiobject, fiber-fed echelle spectrographs M2FS at the Magellan/Clay telescope or Hectochelle at the MMT, reaching a typical limiting magnitude G ≲ 21. Data products include processed spectra from all observations and catalogs listing estimates—derived from template model fitting—of line-of-sight velocity (median uncertainty 1.4 km s−1) effective temperature (255 K), (base-10 logarithm of) surface gravity (0.59 dex in cgs units), [Fe/H] (0.4 dex) and [Mg/Fe] (0.27 dex) abundance ratios. The sample contains multiepoch measurements for 3720 sources, with up to 15 epochs per source, enabling studies of intrinsic spectroscopic variability. The sample contains 6087 likely red giant stars (based on surface gravity), and 4492 likely members (based on line-of-sight velocity and Gaia-measured proper motion) of the target systems. The number of member stars per individual target system ranges from a few, for the faintest systems, to ∼850 for the most luminous. For most systems, our new samples extend over wider fields than have previously been observed; of the likely members in our samples, 820 lie beyond 2 times the projected half-light radius of their host system, and 42 lie beyond 5 R
                  half.}, number={1}, journal={The Astrophysical Journal Supplement Series}, publisher={American Astronomical Society}, author={Walker, Matthew G. and Caldwell, Nelson and Mateo, Mario and Olszewski, Edward W. and Pace, Andrew B. and Bailey, John I., III and Koposov, Sergey E. and Roederer, Ian U.}, year={2023}, month={Aug}, pages={19} }
 @article{ji_simon_roederer_magg_frebel_johnson_klessen_magg_cescutti_mateo_et al._2023, title={Metal Mixing in the r-process Enhanced Ultrafaint Dwarf Galaxy Reticulum II*}, volume={165}, ISSN={0004-6256 1538-3881}, url={http://dx.doi.org/10.3847/1538-3881/acad84}, DOI={10.3847/1538-3881/acad84}, abstractNote={Abstract
               The ultrafaint dwarf galaxy Reticulum II was enriched by a single rare and prolific r-process event. The r-process content of Reticulum II thus provides a unique opportunity to study metal mixing in a relic first galaxy. Using multi-object high-resolution spectroscopy with VLT/GIRAFFE and Magellan/M2FS, we identify 32 clear spectroscopic member stars and measure abundances of Mg, Ca, Fe, and Ba where possible. We find 
                     


                     
                        
                           
                              72
                           
                           
                              −
                              12
                           
                           
                              +
                              10
                           
                        
                        %
                     
                     
                   of the stars are r-process-enhanced, with a mean 
                     


                     
                        
                           
                              [
                              Ba
                              /
                              H
                              ]
                           
                        
                        =
                        −
                        1.68
                        
                        ±
                        
                        0.07
                     
                     
                   and unresolved intrinsic dispersion σ
                  [Ba/H]<0.20. The homogeneous r-process abundances imply that Ret II’s metals are well mixed by the time the r-enhanced stars form, which simulations have shown requires at least 100 Myr of metal mixing in between bursts of star formation to homogenize. This is the first direct evidence of bursty star formation in an ultrafaint dwarf galaxy. The homogeneous dilution prefers a prompt and high-yield r-process site, such as collapsar disk winds or prompt neutron star mergers. We also find evidence from [Ba/H] and [Mg/Ca] that the r-enhanced stars in Ret II formed in the absence of substantial pristine gas accretion, perhaps indicating that ≈70% of Ret II stars formed after reionization.}, number={3}, journal={The Astronomical Journal}, publisher={American Astronomical Society}, author={Ji, Alexander P. and Simon, Joshua D. and Roederer, Ian U. and Magg, Ekaterina and Frebel, Anna and Johnson, Christian I. and Klessen, Ralf S. and Magg, Mattis and Cescutti, Gabriele and Mateo, Mario and et al.}, year={2023}, month={Feb}, pages={100} }
 @article{placco_almeida-fernandes_holmbeck_roederer_mardini_hayes_venn_chiboucas_deibert_gamen_et al._2023, title={SPLUS J142445.34-254247.1: An <i>r</i>-process-enhanced, Actinide-boost, Extremely Metal-poor Star Observed with GHOST}, volume={959}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ad077e}, DOI={10.3847/1538-4357/ad077e}, abstractNote={Abstract
               We report on a chemo-dynamical analysis of SPLUS J142445.34−254247.1 (SPLUS J1424−2542), an extremely metal-poor halo star enhanced in elements formed by the rapid neutron-capture process (r-process). This star was first selected as a metal-poor candidate from its narrowband S-PLUS photometry and followed up spectroscopically in medium resolution with Gemini-South/GMOS, which confirmed its low-metallicity status. High-resolution spectroscopy was gathered with GHOST at Gemini-South, allowing for the determination of the chemical abundances for 36 elements, from carbon to thorium. At [Fe/H] = −3.39, SPLUS J1424−2542 is one of the lowest-metallicity stars with measured Th and has the highest 
                     


                     
                        log
                        ϵ
                        (
                        Th
                        
                           /
                        
                        Eu
                        )
                     
                     
                   observed to date, making it part of the “actinide-boost” category of r-process–enhanced stars. The analysis presented here suggests that the gas cloud from which SPLUS J1424−2542 formed must have been enriched by at least two progenitor populations. The light-element (Z ≤ 30) abundance pattern is consistent with the yields from a supernova explosion of metal-free stars with 11.3–13.4 M
                  ⊙, and the heavy-element (Z ≥ 38) abundance pattern can be reproduced by the yields from a neutron star merger (1.66 M
                  ⊙ and 1.27 M
                  ⊙) event. A kinematical analysis also reveals that SPLUS J1424−2542 is a low-mass, old halo star with a likely in situ origin, not associated with any known early merger events in the Milky Way.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Placco, Vinicius M. and Almeida-Fernandes, Felipe and Holmbeck, Erika M. and Roederer, Ian U. and Mardini, Mohammad K. and Hayes, Christian R. and Venn, Kim and Chiboucas, Kristin and Deibert, Emily and Gamen, Roberto and et al.}, year={2023}, month={Dec} }
 @article{shank_beers_placco_gudin_catapano_holmbeck_ezzeddine_roederer_sakari_frebel_et al._2023, title={The R-Process Alliance: Chemodynamically Tagged Groups. II. An Extended Sample of Halo r-process-enhanced Stars}, volume={943}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aca322}, DOI={10.3847/1538-4357/aca322}, abstractNote={Abstract
               Orbital characteristics based on Gaia Early Data Release 3 astrometric parameters are analyzed for ∼1700 r-process-enhanced (RPE; [Eu/Fe] > +0.3) metal-poor stars ([Fe/H] ≤ −0.8) compiled from the R-Process Alliance, the GALactic Archaeology with HERMES (GALAH) DR3 survey, and additional literature sources. We find dynamical clusters of these stars based on their orbital energies and cylindrical actions using the HDBSCAN unsupervised learning algorithm. We identify 36 chemodynamically tagged groups (CDTGs) containing between five and 22 members; 17 CDTGs have at least 10 member stars. Previously known Milky Way (MW) substructures such as Gaia-Sausage-Enceladus, the splashed disk, the metal-weak thick disk, the Helmi stream, LMS-1 (Wukong), and Thamnos are reidentified. Associations with MW globular clusters are determined for seven CDTGs; no recognized MW dwarf galaxy satellites were associated with any of our CDTGs. Previously identified dynamical groups are also associated with our CDTGs, adding structural determination information and possible new identifications. Carbon-enhanced metal-poor RPE (CEMP-r) stars are identified among the targets; we assign these to morphological groups in a Yoon–Beers A(C)
                     c
                   versus [Fe/H] diagram. Our results confirm previous dynamical analyses that showed RPE stars in CDTGs share common chemical histories, influenced by their birth environments.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Shank, Derek and Beers, Timothy C. and Placco, Vinicius M. and Gudin, Dmitrii and Catapano, Thomas and Holmbeck, Erika M. and Ezzeddine, Rana and Roederer, Ian U. and Sakari, Charli M. and Frebel, Anna and et al.}, year={2023}, month={Jan}, pages={23} }
 @article{pace_koposov_walker_caldwell_mateo_olszewski_roederer_bailey_belokurov_kuehn_et al._2023, title={The kinematics, metallicities, and orbits of six recently discovered Galactic star clusters with Magellan/M2FS spectroscopy}, volume={526}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stad2760}, DOI={10.1093/mnras/stad2760}, abstractNote={ABSTRACT
               We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters (Gran 3/Patchick 125, Gran 4, Garro 01, and LP 866) and two newly discovered open clusters (Gaia 9 and Gaia 10) at low Galactic latitudes. We measure line-of-sight velocities and stellar parameters ([Fe/H], log g, Teff, and [Mg/Fe]) from high-resolution spectroscopy centred on the Mg triplet and identify 20–80 members per star cluster. We determine the kinematics and chemical properties of each cluster and measure the systemic proper motion and orbital properties by utilizing Gaia astrometry. We find Gran 3 to be an old, metal-poor (mean metallicity of [Fe/H] = −1.83) globular cluster located in the Galactic bulge on a retrograde orbit. Gran 4 is an old, metal-poor ([Fe/H] = −1.84) globular cluster with a halo-like orbit that happens to be passing through the Galactic plane. The orbital properties of Gran 4 are consistent with the proposed LMS-1/Wukong and/or Helmi streams merger events. Garro 01 is metal-rich ([Fe/H] = −0.30) and on a near-circular orbit in the outer disc but its classification as an open cluster or globular cluster is ambiguous. Gaia 9 and Gaia 10 are among the most distant known open clusters at $R_{\mathrm{GC}}\sim 18,~21.2~\mathrm{\, kpc}$ and most metal-poor with [Fe/H] ∼−0.50, −0.34 for Gaia 9 and Gaia 10, respectively. LP 866 is a nearby, metal-rich open cluster ([Fe/H] = +0.10). The discovery and confirmation of multiple star clusters in the Galactic plane shows the power of Gaia astrometry and the star cluster census remains incomplete.}, number={1}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Pace, Andrew B and Koposov, Sergey E and Walker, Matthew G and Caldwell, Nelson and Mateo, Mario and Olszewski, Edward W and Roederer, Ian U and Bailey, John I, III and Belokurov, Vasily and Kuehn, Kyler and et al.}, year={2023}, month={Sep}, pages={1075–1094} }
 @article{shah_ezzeddine_ji_hansen_roederer_catelan_hackshaw_holmbeck_beers_surman_2023, title={Uranium Abundances and Ages of r-process Enhanced Stars with Novel U ii Lines*}, volume={948}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/acb8af}, DOI={10.3847/1538-4357/acb8af}, abstractNote={Abstract
               The ages of the oldest stars shed light on the birth, chemical enrichment, and chemical evolution of the universe. Nucleocosmochronometry provides an avenue to determining the ages of these stars independent from stellar-evolution models. The uranium abundance, which can be determined for metal-poor r-process enhanced (RPE) stars, has been known to constitute one of the most robust chronometers known. So far, U abundance determination has used a single U ii line at λ3859 Å. Consequently, U abundance has been reliably determined for only five RPE stars. Here, we present the first homogeneous U abundance analysis of four RPE stars using two novel U ii lines at λ4050 Å and λ4090 Å, in addition to the canonical λ3859 Å line. We find that the U ii lines at λ4050 Å and λ4090 Å are reliable and render U abundances in agreement with the λ3859 U abundance, for all of the stars. We, thus, determine revised U abundances for RPE stars, 2MASS J09544277+5246414, RAVE J203843.2–002333, HE 1523–0901, and CS 31082–001, using multiple U ii lines. We also provide nucleocosmochronometric ages of these stars based on the newly derived U, Th, and Eu abundances. The results of this study open up a new avenue to reliably and homogeneously determine U abundance for a significantly larger number of RPE stars. This will, in turn, enable robust constraints on the nucleocosmochronometric ages of RPE stars, which can be applied to understand the chemical enrichment and evolution in the early universe, especially of r-process elements.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Shah, Shivani P. and Ezzeddine, Rana and Ji, Alexander P. and Hansen, Terese T. and Roederer, Ian U. and Catelan, Márcio and Hackshaw, Zoe and Holmbeck, Erika M. and Beers, Timothy C. and Surman, Rebecca}, year={2023}, month={May}, pages={122} }
 @misc{marconi_abreu_adibekyan_alberti_albrecht_alcaniz_aliverti_allende prieto_alvarado gómez_amado_et al._2022, title={ANDES, the high resolution spectrograph for the ELT: science case, baseline design and path to construction}, url={http://dx.doi.org/10.1117/12.2628689}, DOI={10.1117/12.2628689}, abstractNote={The first generation of ELT instruments includes an optical-infrared high resolution spectrograph, indicated as ELT-HIRES and recently christened ANDES (ArmazoNes high Dispersion Echelle Spectrograph). ANDES consists of three fibre-fed spectrographs (UBV, RIZ, YJH) providing a spectral resolution of ∼100,000 with a minimum simultaneous wavelength coverage of 0.4-1.8 µm with the goal of extending it to 0.35-2.4 µm with the addition of a K band spectrograph. It operates both in seeing- and diffraction-limited conditions and the fibre-feeding allows several, interchangeable observing modes including a single conjugated adaptive optics module and a small diffraction-limited integral field unit in the NIR. Its modularity will ensure that ANDES can be placed entirely on the ELT Nasmyth platform, if enough mass and volume is available, or partly in the Coudé room. ANDES has a wide range of groundbreaking science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Among the top science cases there are the detection of biosignatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars, tests on the stability of Nature’s fundamental couplings, and the direct detection of the cosmic acceleration. The ANDES project is carried forward by a large international consortium, composed of 35 Institutes from 13 countries, forming a team of more than 200 scientists and engineers which represent the majority of the scientific and technical expertise in the field among ESO member states.}, journal={Ground-based and Airborne Instrumentation for Astronomy IX}, publisher={SPIE}, author={Marconi, Alessandro and Abreu, M. and Adibekyan, V. and Alberti, V. and Albrecht, S. and Alcaniz, J. and Aliverti, M. and Allende Prieto, C. and Alvarado Gómez, J. D. and Amado, P. J. and et al.}, editor={Evans, Christopher J. and Bryant, Julia J. and Motohara, KentaroEditors}, year={2022}, month={Aug} }
 @book{kowkabany_ezzeddine_charbonnel_roederer_li_hackshaw_beers_frebel_hansen_holmbeck_et al._2022, title={Discovery of an Ultra Lithium-rich Metal-Poor Red Giant star}, DOI={10.48550/arXiv.2209.02184}, abstractNote={We present the discovery of 2MASS J05241392-0336543 (hereafter J0524-0336), a very metal-poor ([Fe/H]=-2.43 \pm 0.16), highly r-process-enhanced ([Eu/Fe]=+1.34 \pm 0.10) Milky Way halo field red giant star, with an ultra high Li abundance of A(Li)(3D,NLTE)=5.62 \pm 0.25 and [Li/Fe]=+7.00 \pm 0.25, respectively. This makes J0524-0336 the most lithium-enhanced giant star discovered to date. We present a detailed analysis of the star's atmospheric stellar parameters and chemical-abundance determinations. Additionally, we detect infrared excess and variable emission in the wings of the H$_\alpha$ absorption line across multiple epochs, indicative of a potential enhanced mass-loss event with possible outflows. Our analysis reveals that J0524-0336 lies either between the bump and the tip of the Red Giant Branch (RGB), or on the early-Asymptotic Giant Branch (e-AGB). We investigate the possible sources of lithium enrichment in J0524-0336, including both internal and external sources. Based on current models and on the observational evidence we have collected, our study shows that J0524-0336 may be undergoing the so-called lithium flash that is expected to occur in low-mass stars when they reach the RGB bump and/or the early-AGB.}, number={2209.021842209.02184}, author={Kowkabany, J. and Ezzeddine, R. and Charbonnel, C. and Roederer, I.U. and Li, Y. and Hackshaw, Z. and Beers, T.C. and Frebel, A. and Hansen, T.T. and Holmbeck, E. and et al.}, year={2022} }
 @article{schatz_becerril reyes_best_brown_chatziioannou_chipps_deibel_ezzeddine_galloway_hansen_et al._2022, title={Horizons: nuclear astrophysics in the 2020s and Beyond}, volume={49}, ISSN={["1361-6471"]}, DOI={10.1088/1361-6471/ac8890}, abstractNote={Abstract
               Nuclear astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities.}, number={11}, journal={Journal of Physics G: Nuclear and Particle Physics}, author={Schatz, H. and Becerril Reyes, A.D. and Best, A. and Brown, E.F. and Chatziioannou, K. and Chipps, K.A. and Deibel, C.M. and Ezzeddine, R. and Galloway, D.K. and Hansen, C.J. and et al.}, year={2022}, pages={110502} }
 @article{roederer_cowan_pignatari_beers_hartog_ezzeddine_frebel_hansen_holmbeck_mumpower_et al._2022, title={The R-Process Alliance: Abundance Universality among Some Elements at and between the First and Second R-Process Peaks*}, volume={936}, url={https://doi.org/10.3847/1538-4357/ac85bc}, DOI={10.3847/1538-4357/ac85bc}, abstractNote={Abstract
               We present new observational benchmarks of rapid neutron-capture process (r-process) nucleosynthesis for elements at and between the first (A ∼ 80) and second (A ∼ 130) peaks. Our analysis is based on archival ultraviolet and optical spectroscopy of eight metal-poor stars with Se (Z = 34) or Te (Z = 52) detections, whose r-process enhancement varies by more than a factor of 30 (−0.22 ≤ [Eu/Fe] ≤ +1.32). We calculate ratios among the abundances of Se, Sr through Mo (38 ≤ Z ≤ 42), and Te. These benchmarks may offer a new empirical alternative to the predicted solar system r-process residual pattern. The Te abundances in these stars correlate more closely with the lighter r-process elements than the heavier ones, contradicting and superseding previous findings. The small star-to-star dispersion among the abundances of Se, Sr, Y, Zr, Nb, Mo, and Te (≤0.13 dex, or 26%) matches that observed among the abundances of the lanthanides and third r-process-peak elements. The concept of r-process universality that is recognized among the lanthanide and third-peak elements in r-process-enhanced stars may also apply to Se, Sr, Y, Zr, Nb, Mo, and Te, provided the overall abundances of the lighter r-process elements are scaled independently of the heavier ones. The abundance behavior of the elements Ru through Sn (44 ≤ Z ≤ 50) requires further study. Our results suggest that at least one relatively common source in the early Universe produced a consistent abundance pattern among some elements spanning the first and second r-process peaks.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Cowan, John J. and Pignatari, Marco and Beers, Timothy C. and Hartog, Elizabeth A. Den and Ezzeddine, Rana and Frebel, Anna and Hansen, Terese T. and Holmbeck, Erika M. and Mumpower, Matthew R. and et al.}, year={2022}, month={Sep}, pages={84} }
 @article{roederer_lawler_den hartog_placco_surman_beers_ezzeddine_frebel_hansen_hattori_et al._2022, title={The R-process Alliance: A Nearly Complete R-process Abundance Template Derived from Ultraviolet Spectroscopy of the R-process-enhanced Metal-poor Star HD 222925*}, volume={260}, ISSN={0067-0049 1538-4365}, url={http://dx.doi.org/10.3847/1538-4365/ac5cbc}, DOI={10.3847/1538-4365/ac5cbc}, abstractNote={Abstract
               We present a nearly complete rapid neutron-capture process (r-process) chemical inventory of the metal-poor ([Fe/H] = −1.46 ± 0.10) r-process-enhanced ([Eu/Fe] = +1.32 ± 0.08) halo star HD 222925. This abundance set is the most complete for any object beyond the solar system, with a total of 63 metals detected and seven with upper limits. It comprises 42 elements from 31 ≤ Z ≤ 90, including elements rarely detected in r-process-enhanced stars, such as Ga, Ge, As, Se, Cd, In, Sn, Sb, Te, W, Re, Os, Ir, Pt, and Au. We derive these abundances from an analysis of 404 absorption lines in ultraviolet spectra collected using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope and previously analyzed optical spectra. A series of appendices discusses the atomic data and quality of fits for these lines. The r-process elements from Ba to Pb, including all elements at the third r-process peak, exhibit remarkable agreement with the solar r-process residuals, with a standard deviation of the differences of only 0.08 dex (17%). In contrast, deviations among the lighter elements from Ga to Te span nearly 1.4 dex, and they show distinct trends from Ga to Se, Nb through Cd, and In through Te. The r-process contribution to Ga, Ge, and As is small, and Se is the lightest element whose production is dominated by the r-process. The lanthanide fraction, log X
                  La = −1.39 ± 0.09, is typical for r-process-enhanced stars and higher than that of the kilonova from the GW170817 neutron-star merger event. We advocate adopting this pattern as an alternative to the solar r-process-element residuals when confronting future theoretical models of heavy-element nucleosynthesis with observations.}, number={2}, journal={The Astrophysical Journal Supplement Series}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Lawler, James E. and Den Hartog, Elizabeth A. and Placco, Vinicius M. and Surman, Rebecca and Beers, Timothy C. and Ezzeddine, Rana and Frebel, Anna and Hansen, Terese T. and Hattori, Kohei and et al.}, year={2022}, month={Jun}, pages={27} }
 @article{mardini_frebel_ezzeddine_chiti_meiron_ji_placco_roederer_meléndez_2022, title={The chemical abundance pattern of the extremely metal-poor thin disc star 2MASS J1808−5104 and its origins}, volume={517}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stac2783}, DOI={10.1093/mnras/stac2783}, abstractNote={ABSTRACT
               We present a high-resolution (R ∼ 35 000), high signal-to-noise (S/N = 350) Magellan/MIKE spectrum of the bright extremely metal-poor star 2MASS J1808−5104. We find [Fe/H] = −4.01 (spectroscopic LTE stellar parameters), [Fe/H] = −3.8 (photometric stellar parameters), and [Fe/H] = −3.7 (spectroscopic NLTE stellar parameters). We measured a carbon-to-iron ratio of [C/Fe] = 0.38 from the CH G-band. J1808−5104 is thus not carbon-enhanced, contrary to many other stars with similarly low-iron abundances. We also determine, for the first time, a barium abundance ([Ba/Fe] = −0.78), and obtain a significantly reduced upper limit for the nitrogen abundance ([N/Fe] < −0.2). For its [Ba/Fe] abundance, J1808−5104 has a lower [Sr/Ba] ratio compared to other stars, consistent with behaviour of stars in ultra-faint dwarf galaxies. We also fit the abundance pattern of J1808−5104 with nucleosynthesis yields from a grid of Population III supernova models. There is a good fit to the abundance pattern that suggests J1808−5104 originated from gas enriched by a single massive supernova with a high explosion energy of E = 10 × 1051 erg and a progenitor stellar mass of M = 29.5 M⊙. Interestingly, J1808−5104 is a member of the Galactic thin disc, as confirmed by our detailed kinematic analysis and calculated stellar actions and velocities. Finally, we also established the orbital history of J1808−5104 using our time-dependent Galactic potential the ORIENT. J1808−5104 appears to have a stable quasi-circular orbit and been largely confined to the thin disc. This unique orbital history, the star’s very old age (∼13.5 Gyr), and the low [C/Fe] and [Sr/Ba] ratios suggest that J1808−5104 may have formed at the earliest epoch of the hierarchical assembly of the Milky Way, and it is most likely associated with the primordial thin disc.}, number={3}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Mardini, Mohammad K and Frebel, Anna and Ezzeddine, Rana and Chiti, Anirudh and Meiron, Yohai and Ji, Alexander P and Placco, Vinicius M and Roederer, Ian U and Meléndez, Jorge}, year={2022}, month={Sep}, pages={3993–4004} }
 @article{mckenzie_yong_marino_monty_wang_karakas_milone_legnardi_roederer_martell_et al._2022, title={The complex stellar system M 22: confirming abundance variations with high precision differential measurements}, volume={516}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stac2254}, DOI={10.1093/mnras/stac2254}, abstractNote={ABSTRACT
               M 22 (NGC 6656) is a chemically complex globular cluster-like system reported to harbour heavy element abundance variations. However, the extent of these variations and the origin of this cluster is still debated. In this work, we investigate the chemical in-homogeneity of M 22 using differential line-by-line analysis of high-quality (R = 110 000, S/N  = 300 per pixel at 514 nm) VLT/UVES spectra of six carefully chosen red giant branch stars. By achieving abundance uncertainties as low as ∼0.01 dex (∼2 per cent), this high precision data validates the results of previous studies and reveals variations in Fe, Na, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Y, Zr, La, Ce, Nd, Sm, and Eu. Additionally, we can confirm that the cluster hosts two stellar populations with a spread of at least 0.24 dex in [Fe/H] and an average s-process abundance spread of 0.65 dex. In addition to global variations across the cluster, we also find non-negligible variations within each of the two populations, with the more metal-poor population hosting larger spreads in elements heavier than Fe than the metal-rich. We address previous works that do not identify anomalous abundances and relate our findings to our current dynamical understanding of the cluster. Given our results, we suggest that M 22 is either a nuclear star cluster, the product of two merged clusters, or an original building block of the Milky Way.}, number={3}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={McKenzie, M and Yong, D and Marino, A F and Monty, S and Wang, E and Karakas, A I and Milone, A P and Legnardi, M V and Roederer, I U and Martell, S and et al.}, year={2022}, month={Aug}, pages={3515–3531} }
 @article{hartog_lawler_sneden_cowan_roederer_sobeck_2021, title={Atomic Transition Probabilities of Neutral Calcium*}, volume={255}, url={https://doi.org/10.3847/1538-4365/ac04b1}, DOI={10.3847/1538-4365/ac04b1}, abstractNote={Abstract
               The goals of this study are (1) to test the best theoretical transition probabilities for Ca i (a relatively light alkaline earth spectrum) from a modern ab initio calculation using configuration interaction plus many-body perturbation theory against the best modern experimental transition probabilities and (2) to produce as accurate and comprehensive a line list of Ca i transition probabilities as is currently possible based on this comparison. We report new Ca i radiative lifetime measurements from a laser-induced fluorescence experiment and new emission branching fraction measurements from a 0.5 m focal length grating spectrometer with a detector array. We combine these data for upper levels that have both a new lifetime and new branching fractions to report log(gf) values for two multiplets consisting of nine transitions. Detailed comparisons are made between theory and experiment, including the measurements reported herein and a selected set of previously published experimental transition probabilities. We find that modern theory compares favorably to experimental measurements in most instances where such data exist. A final list of 202 recommended transition probabilities is presented, which covers lines of Ca i with wavelengths ranging from 2200 to 10000 Å. These are mostly selected from theory but are augmented with high-quality experimental measurements from this work and from the literature. The recommended transition probabilities are used in a redetermination of the Ca abundance in the Sun and in the metal-poor star HD 84937.}, number={2}, journal={The Astrophysical Journal Supplement Series}, publisher={American Astronomical Society}, author={Hartog, E. A. Den and Lawler, J. E. and Sneden, C. and Cowan, J. J. and Roederer, I. U. and Sobeck, J.}, year={2021}, month={Aug}, pages={27} }
 @article{roederer_lawler_2021, title={Detection of Al ii in the Ultraviolet Spectra of Metal-poor Stars: An Empirical LTE Test of NLTE Aluminum Abundance Calculations*}, volume={912}, url={https://doi.org/10.3847/1538-4357/abf142}, DOI={10.3847/1538-4357/abf142}, abstractNote={Abstract
               We report the detection of an Al ii line at 2669.155 Å in 11 metal-poor stars, using ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We derive Al abundances from this line using a standard abundance analysis, assuming local thermodynamic equilibrium (LTE). The mean [Al/Fe] ratio is −0.06 ± 0.04 (σ = 0.22) for these 11 stars spanning − 3.9 < [Fe/H] < −1.3, or [Al/Fe] = −0.10 ± 0.04 (σ = 0.18) for 9 stars spanning −3.0 < [Fe/H] < −1.3 if two carbon-enhanced stars are excluded. We use these abundances to perform an empirical test of non-LTE (NLTE) abundance corrections predicted for resonance lines of Al i, including the commonly used optical Al i line at 3961 Å. The Al ii line is formed in LTE, and the abundance derived from this line matches that derived from high-excitation Al i lines predicted to have minimal NLTE corrections. The differences between the abundance derived from the Al ii line and the LTE abundance derived from Al i resonance lines are +0.4 to +0.9 dex, which match the predicted NLTE corrections for the Al i resonance lines. We conclude that the NLTE abundance calculations are approximately correct and should be applied to LTE abundances derived from Al i lines.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Lawler, James E.}, year={2021}, month={May}, pages={119} }
 @article{song_mateo_bailey_walker_roederer_olszewski_reiter_kremin_2021, title={Dynamical masses and mass-to-light ratios of resolved massive star clusters – II. Results for 26 star clusters in the Magellanic Clouds}, volume={504}, url={https://doi.org/10.1093/mnras/stab1065}, DOI={10.1093/mnras/stab1065}, abstractNote={ABSTRACT
               We present spectroscopy of individual stars in 26 Magellanic Cloud (MC) star clusters with the aim of estimating dynamical masses and V-band mass-to-light (M/LV) ratios over a wide range in age and metallicity. We obtained 3137 high-resolution stellar spectra with M2FS on the Magellan/Clay Telescope. Combined with 239 published spectroscopic results of comparable quality, we produced a final sample of 2787 stars with good quality spectra for kinematic analysis in the target clusters. Line-of-sight velocities measured from these spectra and stellar positions within each cluster were used in a customized expectation-maximization (EM) technique to estimate cluster membership probabilities. Using appropriate cluster structural parameters and corresponding single-mass dynamical models, this technique ultimately provides self-consistent total mass and M/LV estimates for each cluster. Mean metallicities for the clusters were also obtained and tied to a scale based on calcium IR triplet metallicities. We present trends of the cluster M/LV values with cluster age, mass, and metallicity, and find that our results run about 40 per cent on average lower than the predictions of a set of simple stellar population (SSP) models. Modified SSP models that account for internal and external dynamical effects greatly improve agreement with our results, as can models that adopt a strongly bottom-light IMF. To the extent that dynamical evolution must occur, a modified IMF is not required to match data and models. In contrast, a bottom-heavy IMF is ruled out for our cluster sample as this would lead to higher predicted M/LV values, significantly increasing the discrepancy with our observations.}, number={3}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Song, Ying-Yi and Mateo, Mario and Bailey, John I, III and Walker, Matthew G and Roederer, Ian U and Olszewski, Edward W and Reiter, Megan and Kremin, Anthony}, year={2021}, month={May}, pages={4160–4191} }
 @article{hartog_lawler_roederer_2021, title={Improved Atomic Transition Probabilities for UV and Optical Lines of Hf II and Determination of the Hf Abundance in Two Metal-poor Stars*}, volume={254}, url={https://doi.org/10.3847/1538-4365/abe861}, DOI={10.3847/1538-4365/abe861}, abstractNote={Abstract
               We report new branching fraction measurements for 199 UV and optical transitions of Hf ii. These transitions range in wavelength (wavenumber) from 2068 to 6584 Å (48,322–15,183 cm−1) and originate in 17 odd-parity upper levels ranging in energy from 38,578 to 53,227 cm−1. The branching fractions are combined with radiative lifetimes reported in an earlier study to produce a set of transition probabilities and log(gf) values with accuracy ranging from 5% to 25%. Comparison is made to transition probabilities from the literature where such data exist. We use these new transition probabilities to derive improved Hf abundances in two metal-poor stars. HD 196944 is enhanced in s-process elements, and we derive log ε (Hf) = −0.72 ± 0.03 (σ = 0.09) from 12 Hf ii lines. HD 222925 is enhanced in r-process elements, and we derive log ε (Hf) = 0.32 ± 0.03 (σ = 0.11) from 20 Hf ii lines. These measurements greatly expand the number of potentially useful Hf ii lines for analysis in UV and optical spectra.}, number={1}, journal={The Astrophysical Journal Supplement Series}, publisher={American Astronomical Society}, author={Hartog, E. A. Den and Lawler, J. E. and Roederer, I. U.}, year={2021}, month={May}, pages={5} }
 @article{placco_sneden_roederer_lawler_hartog_hejazi_maas_bernath_2021, title={Linemake: An Atomic and Molecular Line List Generator}, volume={5}, url={https://doi.org/10.3847/2515-5172/abf651}, DOI={10.3847/2515-5172/abf651}, abstractNote={Abstract
               In this research note, we present linemake, an open-source atomic and molecular line list generator. Rather than a replacement for a number of well-established atomic and molecular spectral databases, linemake aims to be a lightweight, easy-to-use tool to generate formatted and curated lists suitable for spectral synthesis work. We encourage users of linemake to understand the sources of their transition data and cite them as appropriate in published work. We provide the code, line database, and an extensive list of literature references in a GitHub repository (https://github.com/vmplacco/linemake), which will be updated regularly as new data become available.}, number={4}, journal={Research Notes of the AAS}, publisher={American Astronomical Society}, author={Placco, Vinicius M. and Sneden, Christopher and Roederer, Ian U. and Lawler, James E. and Hartog, Elizabeth A. Den and Hejazi, Neda and Maas, Zachary and Bernath, Peter}, year={2021}, month={Apr}, pages={92} }
 @article{lipartito_bailey_brandt_mazin_mateo_spencer_roederer_2021, title={Orbital Parameters and Binary Properties of 37 FGK Stars in the Cores of Open Clusters NGC 2516 and NGC 2422}, volume={162}, url={https://doi.org/10.3847/1538-3881/ac2ccd}, DOI={10.3847/1538-3881/ac2ccd}, abstractNote={Abstract
               We present orbits for 24 binaries in the field of open cluster NGC 2516 (∼150 Myr) and 13 binaries in the field of open cluster NGC 2422 (∼130 Myr) using results from a multiyear radial-velocity (RV) survey of the cluster cores. Six of these systems are double-lined spectroscopic binaries. We fit these RV variable systems with orvara, a MCMC-based fitting program that models Keplerian orbits. We use precise stellar parallaxes and proper motions from Gaia EDR3 to determine cluster membership. We impose a barycentric RV prior on all cluster members; this significantly improves our orbital constraints. Two of our systems have periods between five and 15 days, the critical window in which tides efficiently damp orbital eccentricity. These binaries should be included in future analyses of circularization across similarly-aged clusters. We also find a relatively flat distribution of binary mass ratios, consistent with previous work. With the inclusion of TESS light curves for all available targets, we identity target 378–036252 as a new eclipsing binary. We also identify a field star whose secondary has a mass in the brown dwarf range, as well as two cluster members whose RVs suggest the presence of an additional companion. Our orbital fits will help constrain the binary fraction and binary properties across stellar age and across stellar environment.}, number={6}, journal={The Astronomical Journal}, publisher={American Astronomical Society}, author={Lipartito, Isabel and Bailey, John I., III and Brandt, Timothy D. and Mazin, Benjamin A. and Mateo, Mario and Spencer, Meghin E. and Roederer, Ian U.}, year={2021}, month={Dec}, pages={285} }
 @article{yu_li_qu_roederer_bregman_fan_fang_johnson_wang_yang_2021, title={Probing the He <scp>ii</scp> re-Ionization ERa via Absorbing C <scp>iv</scp> Historical Yield (HIERACHY) I: A strong outflow from a <i>z</i> ∼ 4.7 quasar}, volume={505}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stab1614}, DOI={10.1093/mnras/stab1614}, abstractNote={ABSTRACT
               Outflows from supermassive black holes (SMBHs) play an important role in the co-evolution of themselves, their host galaxies, and the larger scale environments. Such outflows are often characterized by emission and absorption lines in various bands and in a wide velocity range blueshifted from the systematic redshift of the host quasar. In this paper, we report a strong broad line region (BLR) outflow from the z ≈ 4.7 quasar BR 1202-0725 based on the high-resolution optical spectrum taken with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph installed on the 6.5 m Magellan/Clay telescope, obtained from the ‘Probing the He ii re-Ionization ERa via Absorbing C iv Historical Yield’ (HIERACHY) project. This rest-frame ultraviolet (UV) spectrum is characterized by a few significantly blueshifted broad emission lines from high ions; the most significant one is the C iv line at a velocity of $\sim -6500$ km s−1 relative to the H α emission line, which is among the highest velocity BLR outflows in observed quasars at z > 4. The measured properties of UV emission lines from different ions, except for O i and Ly α, also follow a clear trend that higher ions tend to be broader and outflow at higher average velocities. There are multiple C iv and Si iv absorbing components identified on the blue wings of the corresponding emission lines, which may be produced by either the outflow or the intervening absorbers.}, number={3}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Yu, Xiaodi and Li, Jiang-Tao and Qu, Zhijie and Roederer, Ian U and Bregman, Joel N and Fan, Xiaohui and Fang, Taotao and Johnson, Sean D and Wang, Feige and Yang, Jinyi}, year={2021}, month={Jun}, pages={4444–4455} }
 @article{gull_frebel_hinojosa_roederer_ji_brauer_2021, title={R-process-rich Stellar Streams in the Milky Way*}, volume={912}, url={https://doi.org/10.3847/1538-4357/abea1a}, DOI={10.3847/1538-4357/abea1a}, abstractNote={Abstract
               We present high-resolution Magellan/MIKE spectra of 22 bright (9 < V < 13.5) metal-poor stars (−3.18 < [Fe/H] < −1.37) in three different stellar streams, the Helmi debris stream, the Helmi trail stream, and the ω Centauri progenitor stream. We augment our Helmi debris sample with results for 10 stars by Roederer et al. for a total of 32 stars. Detailed chemical abundances of light elements as well as heavy neutron-capture elements have been determined for our 22 stars. All three streams contain carbon-enhanced stars. For 13 stars, neutron-capture element lines were detectable, and they all show signatures in agreement with the scaled solar r-process pattern, albeit with a large spread of −0.5 < [Eu/Fe] < +1.3. Eight of these stars show an additional small s-process contribution superposed onto their r-process pattern. This could be discerned because of the relatively high signal-to-noise ratio of the spectra given that the stars are close by in the halo. Our results suggest that the progenitors of these streams experienced one or more r-process events early on, such as a neutron star merger or another prolific r-process source. This widely enriched these host systems before their accretion by the Milky Way. The small s-process contribution suggests the presence of asymptotic giant branch stars and associated local (inhomogeneous) enrichment as part of the ongoing chemical evolution by low-mass stars. Stars in stellar streams may thus be a promising avenue for studying the detailed history of large dwarf galaxies and their role in halo assembly with easily accessible targets for high-quality spectra of many stars.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Gull, Maude and Frebel, Anna and Hinojosa, Karina and Roederer, Ian U. and Ji, Alexander P. and Brauer, Kaley}, year={2021}, month={May}, pages={52} }
 @article{placco_roederer_lee_almeida-fernandes_herpich_perottoni_schoenell_ribeiro_kanaan_2021, title={SPLUS J210428.01−004934.2: An Ultra Metal-poor Star Identified from Narrowband Photometry*}, volume={912}, url={https://doi.org/10.3847/2041-8213/abf93d}, DOI={10.3847/2041-8213/abf93d}, abstractNote={Abstract
               We report on the discovery of SPLUS J210428.01−004934.2, an ultra metal-poor (UMP) star first identified from the narrowband photometry of the Southern Photometric Local Universe Survey (S-PLUS) Data Release 1, in the SDSS Stripe 82 region. Follow-up medium- and high-resolution spectroscopy (with Gemini South and Magellan-Clay, respectively) confirmed the effectiveness of the search for low-metallicity stars using the S-PLUS narrowband photometry. At [Fe/H] = −4.03, SPLUS J2104−0049 has the lowest detected carbon abundance, A(C) = +4.34, when compared to the 34 previously known UMP stars in the literature, which is an important constraint on its stellar progenitor and also on stellar evolution models at the lowest metallicities. Based on its chemical abundance pattern, we speculate that SPLUS J2104−0049 could be a bona fide second-generation star, formed from a gas cloud polluted by a single metal-free ∼ 30M
                  ⊙ star. This discovery opens the possibility of finding additional UMP stars directly from narrowband photometric surveys, a potentially powerful method to help complete the inventory of such peculiar objects in our Galaxy.}, number={2}, journal={The Astrophysical Journal Letters}, publisher={American Astronomical Society}, author={Placco, Vinicius M. and Roederer, Ian U. and Lee, Young Sun and Almeida-Fernandes, Felipe and Herpich, Fábio R. and Perottoni, Hélio D. and Schoenell, William and Ribeiro, Tiago and Kanaan, Antonio}, year={2021}, month={May}, pages={L32} }
 @article{gudin_shank_beers_yuan_limberg_roederer_placco_holmbeck_dietz_rasmussen_et al._2021, title={The R-Process Alliance: Chemodynamically Tagged Groups of Halo r-process-enhanced Stars Reveal a Shared Chemical-evolution History}, volume={908}, url={https://doi.org/10.3847/1538-4357/abd7ed}, DOI={10.3847/1538-4357/abd7ed}, abstractNote={Abstract
               We derive dynamical parameters for a large sample of 446 r-process-enhanced (RPE) metal-poor stars in the halo and disk systems of the Milky Way, based on data releases from the R-Process Alliance, supplemented by additional literature samples. This sample represents more than a 10-fold increase in size relative to that previously considered by Roederer et al. and, by design, covers a larger range of r-process-element enrichment levels. We test a number of clustering analysis methods on the derived orbital energies and other dynamical parameters for this sample, ultimately deciding on application of the HDBSCAN algorithm, which obtains 30 individual chemodynamically tagged groups (CDTGs); 21 contain between 3 and 5 stars, and 9 contain between 6 and 12 stars. Even though the clustering was performed solely on the basis of their dynamical properties, the stars in these CDTGs exhibit statistically significant similarities in their metallicity ([Fe/H]), carbonicity ([C/Fe]), and neutron-capture element ratios ([Sr/Fe], [Ba/Fe], and [Eu/Fe]). These results demonstrate that the RPE stars in these CDTGs have likely experienced common chemical-evolution histories, presumably in their parent satellite galaxies or globular clusters, prior to being disrupted into the Milky Way’s halo. We also confirm the previous claim that the orbits of the RPE stars preferentially exhibit pericentric distances that are substantially lower than the present distances of surviving ultrafaint dwarf and canonical dwarf spheroidal galaxies, consistent with the disruption hypothesis. The derived dynamical parameters for several of our CDTGs indicate their association with previously known substructures, dynamically tagged groups, and RPE groups.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Gudin, Dmitrii and Shank, Derek and Beers, Timothy C. and Yuan, Zhen and Limberg, Guilherme and Roederer, Ian U. and Placco, Vinicius and Holmbeck, Erika M. and Dietz, Sarah and Rasmussen, Kaitlin C. and et al.}, year={2021}, month={Feb}, pages={79} }
 @article{cowan_sneden_roederer_lawler_hartog_sobeck_boesgaard_2020, title={Detailed Iron-peak Element Abundances in Three Very Metal-poor Stars}, volume={890}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/ab6aa9}, DOI={10.3847/1538-4357/ab6aa9}, abstractNote={Abstract
               We have obtained new detailed abundances of the Fe-group elements Sc through Zn (Z = 21–30) in three very metal-poor ([Fe/H] ≈ −3) stars: BD+03o740, BD−13o3442, and CD−33o1173. High-resolution ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra in the wavelength range 2300–3050 Å were gathered, and complemented by an assortment of optical echelle spectra. The analysis featured recent laboratory atomic data for a number of neutral and ionized species for all Fe-group elements except Cu and Zn. A detailed examination of scandium, titanium, and vanadium abundances in large-sample spectroscopic surveys indicates that they are positively correlated in stars with [Fe/H] ≤ −2. The abundances of these elements in BD+03o740, BD−13o3442, CD−33o1173, and HD 84937 (studied in a previous paper of this series) are in accord with these trends and lie at the high end of the correlations. Six elements have detectable neutral and ionized features, and generally their abundances are in reasonable agreement. For Cr we find only minimal abundance disagreement between the neutral (mean of [Cr i/Fe] = +0.01) and ionized species (mean of [Cr ii/Fe] = +0.08), unlike most studies in the past. The prominent exception is Co, for which the neutral species indicates a significant overabundance (mean of [Co i/H] = −2.53), while no such enhancement is seen for the ionized species (mean of [Co ii/H] = −2.93). These new stellar abundances, especially the correlations among Sc, Ti, and V, suggest that models of element production in early high-mass metal-poor stars should be revisited.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Cowan, John J. and Sneden, Christopher and Roederer, Ian U. and Lawler, James E. and Hartog, Elizabeth A. Den and Sobeck, Jennifer S. and Boesgaard, Ann Merchant}, year={2020}, month={Feb}, pages={119} }
 @article{roederer_lawler_holmbeck_beers_ezzeddine_frebel_hansen_ivans_karakas_placco_et al._2020, title={Detection of Pb II in the Ultraviolet Spectra of Three Metal-poor Stars}, volume={902}, ISSN={2041-8213}, url={http://dx.doi.org/10.3847/2041-8213/abbc21}, DOI={10.3847/2041-8213/abbc21}, abstractNote={Abstract
               We report the first detection of the Pb ii line at 2203.534 Å in three metal-poor stars, using ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We perform a standard abundance analysis assuming local thermodynamic equilibrium (LTE) to derive lead (Pb, Z = 82) abundances. The Pb ii line yields a higher abundance than Pb i lines by +0.36 ± 0.34 dex and +0.49 ± 0.28 dex in the stars HD 94028 and HD 196944, where Pb i lines had been detected previously. The Pb ii line is likely formed in LTE, and these offsets affirm previous calculations showing that Pb i lines commonly used as abundance indicators underestimate the Pb abundance in LTE. Pb is enhanced in the s-process-enriched stars HD 94028 ([Pb/Fe] = +0.95 ± 0.14) and HD 196944 ([Pb/Fe] = +2.28 ± 0.23), and we show that 208Pb is the dominant Pb isotope in these two stars. The 
                     


                     
                  (Pb/Eu) ratio in the r-process-enhanced star HD 222925 is 0.76 ± 0.14, which matches the solar system r-process ratio and indicates that the solar system r-process residuals for Pb are, in aggregate, correct. The Th/Pb chronometer in HD 222925 yields an age of 8.2 ± 5.8 Gyr, and we highlight the potential of the Th/Pb chronometer as a relatively model-insensitive age indicator in r-process-enhanced stars.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Lawler, James E. and Holmbeck, Erika M. and Beers, Timothy C. and Ezzeddine, Rana and Frebel, Anna and Hansen, Terese T. and Ivans, Inese I. and Karakas, Amanda I. and Placco, Vinicius M. and et al.}, year={2020}, month={Oct}, pages={L24} }
 @article{den hartog_lawler_roederer_2020, title={Hyperfine Structure Constants for Levels of <sup>175}, volume={248}, ISSN={1538-4365}, url={http://dx.doi.org/10.3847/1538-4365/ab84f5}, DOI={10.3847/1538-4365/ab84f5}, abstractNote={Accurate and complete atomic data are required for the determination of accurate stellar photospheric abundances. This includes hyperfine structure (HFS) for some elements—those with high nuclear spin and/or large nuclear magnetic moments. Lutetium is one such element. In this study, spectra of a commercial Lu–Ne hollow cathode lamp operating at a low current were recorded with the high-resolution University of Wisconsin 3 m focal length echelle spectrograph. These spectra of resolved and partially resolved hyperfine patterns for 35 ultraviolet and blue transitions of Lu ii have been studied for the purpose of extracting hyperfine constants. We present HFS constants for 16 levels of singly ionized lutetium, 10 of which have been measured for the first time.}, number={1}, journal={The Astrophysical Journal Supplement Series}, publisher={American Astronomical Society}, author={Den Hartog, E. A. and Lawler, J. E. and Roederer, I. U.}, year={2020}, month={Apr}, pages={10} }
 @article{cain_frebel_ji_placco_ezzeddine_roederer_hattori_beers_meléndez_hansen_et al._2020, title={The <i>R</i>-Process Alliance: A Very Metal-poor, Extremely <i>r</i>-process-enhanced Star with [Eu/Fe] = + 2.2, and the Class of <i>r-</i>III Stars}, volume={898}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/ab97ba}, DOI={10.3847/1538-4357/ab97ba}, abstractNote={Abstract
               We report the discovery of J1521−3538, a bright (V = 12.2), very metal-poor ([Fe/H] = −2.8) strongly r-process-enhanced field horizontal branch star, based on a high-resolution, high signal-to-noise Magellan/MIKE spectrum. J1521−3538 shows the largest r-process element overabundance in any known r-process-enhanced star, with [Eu/Fe] = +2.2, and its chemical abundances of 22 neutron-capture elements closely match the scaled solar r-process pattern. J1521−3538 is also one of few known carbon-enhanced metal-poor stars with r-process enhancement (CEMP-r stars), as found after correcting the measured C abundance for the star’s evolutionary status. We propose to extend the existing classification of moderately enhanced (
                     


                     
                  ) r-I and strongly r-process enhanced (
                     


                     
                  ) r-II stars to include an r-III class, for r-process stars such as J1521−3538, with 
                     


                     
                   and 
                     


                     
                  , or 
                     


                     
                   times the solar ratio of europium to iron. Using cosmochronometry, we estimate J1521−3538 to be 
                     


                     
                   and 
                     


                     
                  , using two different sets of initial production ratios. These ages are based on measurements of the Th line at 4019 Å and other r-process element abundances. This is broadly consistent with the old age of a low-mass, metal-poor field red horizontal branch star. J1521−3538 likely originated in a low-mass dwarf galaxy that was later accreted by the Milky Way, as evidenced by its highly eccentric orbit.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Cain, Madelyn and Frebel, Anna and Ji, Alexander P. and Placco, Vinicius M. and Ezzeddine, Rana and Roederer, Ian U. and Hattori, Kohei and Beers, Timothy C. and Meléndez, Jorge and Hansen, Terese T. and et al.}, year={2020}, month={Jul}, pages={40} }
 @article{ezzeddine_rasmussen_frebel_chiti_hinojisa_placco_ji_beers_hansen_roederer_et al._2020, title={The <i>R</i>-Process Alliance: First Magellan/MIKE Release from the Southern Search for <i>R</i>-process-enhanced Stars}, volume={898}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/ab9d1a}, DOI={10.3847/1538-4357/ab9d1a}, abstractNote={Abstract
               Extensive progress has recently been made in our understanding of heavy-element production via the r-process in the universe, specifically with the first observed neutron star binary merger (NSBM) event associated with the gravitational-wave signal detected by LIGO, GW170817. The chemical abundance patterns of metal-poor r-process-enhanced stars provide key evidence for the dominant site(s) of the r-process and whether NSBMs are sufficiently frequent or prolific r-process sources to be responsible for the majority of r-process material in the universe. We present atmospheric stellar parameters (using a nonlocal thermodynamic equilibrium analysis) and abundances from a detailed analysis of 141 metal-poor stars carried out as part of the R-Process Alliance (RPA) effort. We obtained high-resolution “snapshot” spectroscopy of the stars using the MIKE spectrograph on the 6.5 m Magellan Clay telescope at Las Campanas Observatory in Chile. We find 10 new highly enhanced r-II (with [Eu/Fe] > +1.0), 62 new moderately enhanced r-I (+0.3 < [Eu/Fe] ≤ +1.0), and 17 new limited-r ([Eu/Fe] < +0.3) stars. Among those, we find 17 new carbon-enhanced metal-poor (CEMP) stars, of which five are CEMP-no. We also identify one new s-process-enhanced ([Ba/Eu] > +0.5) and five new r/s (0.0 < [Ba/Eu] < +0.5) stars. In the process, we discover a new ultra-metal-poor (UMP) star at [Fe/H] = −4.02. One of the r-II stars shows a deficit in α and Fe-peak elements, typical of dwarf galaxy stars. Our search for r-process-enhanced stars by RPA efforts has already roughly doubled the known r-process sample.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Ezzeddine, Rana and Rasmussen, Kaitlin and Frebel, Anna and Chiti, Anirudh and Hinojisa, Karina and Placco, Vinicius M. and Ji, Alexander P. and Beers, Timothy C. and Hansen, Terese T. and Roederer, Ian U. and et al.}, year={2020}, month={Aug}, pages={150} }
 @inproceedings{roederer_2020, title={The Environment of the <i>r</i>-process: New Advances Enabled by the Study of the Orbits of <i>r</i>-process-enhanced Stars}, url={http://dx.doi.org/10.7566/jpscp.31.011008}, DOI={10.7566/jpscp.31.011008}, booktitle={Proceedings of the 15th International Symposium on Origin of Matter and Evolution of Galaxies (OMEG15)}, publisher={Journal of the Physical Society of Japan}, author={Roederer, Ian U.}, year={2020}, month={Mar} }
 @book{seager_mennesson_roederer_2020, title={The Habitable Exoplanet Observatory (HabEx) Mission Concept Study Final Report}, DOI={10.48550/arXiv.2001.06683}, abstractNote={The Habitable Exoplanet Observatory, or HabEx, has been designed to be the Great Observatory of the 2030s. For the first time in human history, technologies have matured sufficiently to enable an affordable space-based telescope mission capable of discovering and characterizing Earthlike planets orbiting nearby bright sunlike stars in order to search for signs of habitability and biosignatures. Such a mission can also be equipped with instrumentation that will enable broad and exciting general astrophysics and planetary science not possible from current or planned facilities. HabEx is a space telescope with unique imaging and multi-object spectroscopic capabilities at wavelengths ranging from ultraviolet (UV) to near-IR. These capabilities allow for a broad suite of compelling science that cuts across the entire NASA astrophysics portfolio. HabEx has three primary science goals: (1) Seek out nearby worlds and explore their habitability; (2) Map out nearby planetary systems and understand the diversity of the worlds they contain; (3) Enable new explorations of astrophysical systems from our own solar system to external galaxies by extending our reach in the UV through near-IR. This Great Observatory science will be selected through a competed GO program, and will account for about 50% of the HabEx primary mission. The preferred HabEx architecture is a 4m, monolithic, off-axis telescope that is diffraction-limited at 0.4 microns and is in an L2 orbit. HabEx employs two starlight suppression systems: a coronagraph and a starshade, each with their own dedicated instrument.}, number={2001.066832001.06683}, author={Seager, B.S. and Mennesson, S. and Roederer, I.U.}, year={2020} }
 @article{holmbeck_hansen_beers_placco_whitten_rasmussen_roederer_ezzeddine_sakari_frebel_et al._2020, title={The R-Process Alliance: Fourth Data Release from the Search for R-process-enhanced Stars in the Galactic Halo*}, url={https://doi.org/10.3847/1538-4365/ab9c19}, DOI={10.3847/1538-4365/ab9c19}, abstractNote={This compilation is the fourth data release from the R-Process Alliance (RPA) search for r-process-enhanced stars and the second release based on “snapshot” high-resolution (R ∼ 30,000) spectra collected with the du Pont 2.5 m Telescope. In this data release, we propose a new delineation between the r-I and r-II stellar classes at , instead of the empirically chosen level previously in use, based on statistical tests of the complete set of RPA data released to date. We also statistically justify the minimum level of [Eu/Fe] for definition of the r-I stars, [Eu/Fe] > +0.3. Redefining the separation between r-I and r-II stars will aid in the analysis of the possible progenitors of these two classes of stars and determine whether these signatures arise from separate astrophysical sources at all. Applying this redefinition to previous RPA data, the number of identified r-II and r-I stars changes to 51 and 121, respectively, from the initial set of data releases published thus far. In this data release, we identify 21 new r-II, 111 new r-I (plus 3 re-identified), and 7 new (plus 1 re-identified) limited-r stars out of a total of 232 target stars, resulting in a total sample of 72 new r-II stars, 232 new r-I stars, and 42 new limited-r stars identified by the RPA to date.}, journal={The Astrophysical Journal Supplement Series}, author={Holmbeck, Erika M. and Hansen, Terese T. and Beers, Timothy C. and Placco, Vinicius M. and Whitten, Devin D. and Rasmussen, Kaitlin C. and Roederer, Ian U. and Ezzeddine, Rana and Sakari, Charli M. and Frebel, Anna and et al.}, year={2020}, month={Aug} }
 @article{roederer_2020, title={The R-Process Alliance: a new stellar template for the r-process abundance pattern}, volume={52}, number={3}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2020}, month={Jun}, pages={207.03} }
 @article{placco_santucci_yuan_mardini_holmbeck_wang_surman_hansen_roederer_beers_et al._2020, title={The R-process Alliance: The Peculiar Chemical Abundance Pattern of RAVE J183013.5−455510*}, url={https://doi.org/10.3847/1538-4357/ab99c6}, DOI={10.3847/1538-4357/ab99c6}, abstractNote={Abstract
               We report on the spectroscopic analysis of RAVE J183013.5−455510, an extremely metal-poor star, highly enhanced in CNO, and with discernible contributions from the rapid neutron-capture process. There is no evidence of binarity for this object. At 
                     


                     
                   = −3.57, this star has one of the lowest metallicities currently observed, with 18 measured abundances of neutron-capture elements. The presence of Ba, La, and Ce abundances above the solar system r-process predictions suggests that there must have been a non-standard source of r-process elements operating at such low metallicities. One plausible explanation is that this enhancement originates from material ejected at unusually high velocities in a neutron star merger event. We also explore the possibility that the neutron-capture elements were produced during the evolution and explosion of a rotating massive star. In addition, based on comparisons with yields from zero-metallicity faint supernova, we speculate that RAVE J1830−4555 was formed from a gas cloud pre-enriched by both progenitor types. From analysis based on Gaia DR2 measurements, we show that this star has orbital properties similar to the Galactic metal-weak thick-disk stellar population.}, journal={The Astrophysical Journal}, author={Placco, Vinicius M. and Santucci, Rafael M. and Yuan, Zhen and Mardini, Mohammad K. and Holmbeck, Erika M. and Wang, Xilu and Surman, Rebecca and Hansen, Terese T. and Roederer, Ian U. and Beers, Timothy C. and et al.}, year={2020}, month={Jul} }
 @article{ou_roederer_sneden_cowan_lawler_shectman_thompson_2020, title={Vanadium Abundance Derivations in 255 Metal-poor Stars}, volume={900}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/abaa50}, DOI={10.3847/1538-4357/abaa50}, abstractNote={Abstract
               We present vanadium (V) abundances for 255 metal-poor stars derived from high-resolution optical spectra from the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert G. Tull Coudé Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory, and the High Resolution Spectrograph on the Hobby–Eberly Telescope at McDonald Observatory. We use updated V i and V ii atomic transition data from recent laboratory studies, and we increase the number of lines examined (from one to four lines of V i, and from two to seven lines of V ii). As a result, we reduce the V abundance uncertainties for most stars by more than 20% and expand the number of stars with V detections from 204 to 255. In the metallicity range −4.0 < [Fe/H] < −1.0, we calculate the mean ratios [V i/Fe i] = −0.10 ± 0.01 (σ = 0.16) from 128 stars with ≥2 V i lines detected, [V ii/Fe ii] = +0.13 ± 0.01 (σ = 0.16) from 220 stars with ≥2 V ii lines detected, and [V ii/V i] = +0.25 ± 0.01 (σ = 0.15) from 119 stars. We suspect that this offset is due to departures from local thermodynamic equilibrium, and we recommend using [V ii/Fe ii], which is enhanced relative to the solar ratio, as a better representation of [V/Fe]. We provide more extensive evidence for abundance correlations detected previously among scandium, titanium, and vanadium, and we identify no systematic effects in the analysis that can explain these correlations.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Ou, Xiaowei and Roederer, Ian U. and Sneden, Christopher and Cowan, John J. and Lawler, James E. and Shectman, Stephen A. and Thompson, Ian B.}, year={2020}, month={Sep}, pages={106} }
 @article{song_mateo_mackey_olszewski_roederer_walker_bailey_2019, title={Dynamical masses and mass-to-light ratios of resolved massive star clusters – I. NGC 419 and NGC 1846}, volume={490}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stz2502}, DOI={10.1093/mnras/stz2502}, abstractNote={ABSTRACT
               As an introduction of a kinematic survey of Magellanic Cloud (MC) star clusters, we report on the dynamical masses and mass-to-light ratios (M/L) of NGC 419 (Small Magellanic Cloud) and NGC 1846 (Large Magellanic Cloud). We have obtained more than one hundred high-resolution stellar spectra in and around each cluster using the multi-object spectrograph M2FS on the Magellan/Clay Telescope. Line-of-sight velocities and positions of the stars observed in each cluster were used as input to an expectation-maximization algorithm used to estimate cluster membership probabilities, resulting in samples of 46 and 52 likely members (PM ≥ 50 per cent) in NGC 419 and NGC 1846, respectively. This process employed single-mass King models constrained by the structural parameters of the clusters and provided self-consistent dynamical mass estimates for both clusters. Our best-fitting results show that NGC 419 has a projected central velocity dispersion of $2.44^{+0.37}_{-0.21}$ km s−1, corresponding to a total mass of $7.6^{+2.5}_{-1.3}\times 10^4\ {\rm M}_{\odot }$ and V-band M/L ratio of $0.22^{+0.08}_{-0.05}$ in solar units. For NGC 1846, the corresponding results are $2.04^{+0.28}_{-0.24}$ km s−1, $5.4^{+1.5}_{-1.4}\times 10^4\ {\rm M}_{\odot }$, and $0.32^{+0.11}_{-0.11}$. The mean metallicities of NGC 419 and NGC 1846 are found to be $\rm [Fe/H]=-0.84\pm 0.19$ and −0.70 ± 0.08, respectively, based on the spectra of likely cluster members. We find marginal statistical evidence of rotation in both clusters, though in neither cluster does rotation alter our mass estimates significantly. We critically compare our findings with those of previous kinematic studies of these two clusters in order to evaluate the consistency of our observational results and analytic tools.}, number={1}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Song, Ying-Yi and Mateo, Mario and Mackey, A D and Olszewski, Edward W and Roederer, Ian U and Walker, Matthew G and Bailey, John I, III}, year={2019}, month={Sep}, pages={385–407} }
 @article{ezzeddine_frebel_roederer_tominaga_tumlinson_ishigaki_nomoto_placco_aoki_2019, title={Evidence for an Aspherical Population III Supernova Explosion Inferred from the Hyper-metal-poor Star HE 1327–2326∗}, url={https://doi.org/10.3847/1538-4357/ab14e7}, DOI={10.3847/1538-4357/ab14e7}, abstractNote={Abstract
               We present observational evidence that an aspherical supernova explosion could have occurred in the first stars in the early universe. Our results are based on the first determination of a Zn abundance in a Hubble Space Telescope/Cosmic Origins Spectrograph high-resolution UV spectrum of a hyper-metal-poor (HMP) star, HE 1327−2326, with 
                     


                     
                  . We determine [Zn/Fe] = 0.80 ± 0.25 from a UV Zn i line at 2138 Å, detected at 3.4σ. Yields of a 25 M
                  ⊙ aspherical supernova model with artificially modified densities exploding with E = 5 × 1051 erg best match the entire abundance pattern of HE 1327−2326. Such high-entropy hypernova explosions are expected to produce bipolar outflows, which could facilitate the external enrichment of small neighboring galaxies. This has already been predicted by theoretical studies of the earliest star-forming minihalos. Such a scenario would have significant implications for the chemical enrichment across the early universe, as HMP carbon-enhanced metal-poor (CEMP) stars such as HE 1327−2326 might have formed in such externally enriched environments.}, journal={The Astrophysical Journal}, author={Ezzeddine, Rana and Frebel, Anna and Roederer, Ian U. and Tominaga, Nozomu and Tumlinson, Jason and Ishigaki, Miho and Nomoto, Ken’ichi and Placco, Vinicius M. and Aoki, Wako}, year={2019}, month={May} }
 @article{buzasi_bautista_cummings_mace_roederer_white_2019, title={Fundamental Stellar Physics throughout the Galaxy}, volume={51}, number={3197}, journal={Bulletin of the American Astronomical Society}, author={Buzasi, D. and Bautista, M. and Cummings, J. and Mace, G. and Roederer, I.U. and White, R.}, year={2019}, pages={197} }
 @article{roederer_gnedin_2019, title={High-resolution Optical Spectroscopy of Stars in the Sylgr Stellar Stream}, volume={883}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/ab365c}, DOI={10.3847/1538-4357/ab365c}, abstractNote={Abstract
               We observe two metal-poor main-sequence stars that are members of the recently discovered Sylgr stellar stream. We present radial velocities, stellar parameters, and abundances for 13 elements derived from high-resolution optical spectra collected using the Magellan Inamori Kyocera Echelle spectrograph. The two stars have identical compositions (within 0.13 dex or 1.2σ) among all elements detected. Both stars are very metal-poor ([Fe/H] = −2.92 ± 0.06). Neither star is highly enhanced in C ([C/Fe] < +1.0). Both stars are enhanced in the α elements Mg, Si, and Ca ([α/Fe] = +0.32 ± 0.06), and the ratios among Na, Al, and all Fe-group elements are typical for other stars in the halo and ultra-faint and dwarf spheroidal galaxies at this metallicity. Sr is mildly enhanced ([Sr/Fe] = +0.22 ± 0.11), but Ba is not enhanced ([Ba/Fe] < −0.4), indicating that these stars do not contain high levels of neutron-capture elements. The Li abundances match those found in metal-poor unevolved field stars and globular clusters (GCs) (log ϵ(Li) = 2.05 ± 0.07), which implies that environment is not a dominant factor in determining the Li content of metal-poor stars. The chemical compositions of these two stars cannot distinguish whether the progenitor of the Sylgr stream was a dwarf galaxy or a GC. If the progenitor was a dwarf galaxy, the stream may originate from a dense region such as a nuclear star cluster. If the progenitor was a GC, it would be the most metal-poor GC known.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Gnedin, Oleg Y.}, year={2019}, month={Sep}, pages={84} }
 @article{ji_beaton_chakrabarti_duggan_frebel_geha_hosek_kirby_li_roederer_et al._2019, title={Local Dwarf Galaxy Archaeology}, volume={51}, number={3166}, journal={Bulletin of the American Astronomical Society}, author={Ji, A. and Beaton, R. and Chakrabarti, S. and Duggan, G. and Frebel, A. and Geha, M. and Hosek, M., Jr and Kirby, E. and Li, T. and Roederer, I.U. and et al.}, year={2019}, pages={166} }
 @article{hattori_valluri_castro_roederer_mahler_khullar_2019, title={Origin of a Massive Hyper-runaway Subgiant Star LAMOST-HVS1: Implication from <i>Gaia</i> and Follow-up Spectroscopy}, volume={873}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/ab05c8}, DOI={10.3847/1538-4357/ab05c8}, abstractNote={Abstract
               We report that LAMOST-HVS1 is a massive hyper-runaway subgiant star with a mass of 8.3 
                     


                     
                   and super-solar metallicity, which was ejected from the inner stellar disk of the Milky Way ∼33 
                     


                     
                   ago with the intrinsic ejection velocity of 
                     


                     
                   (corrected for the streaming motion of the disk), based on the proper motion data from Gaia Data Release 2 (DR2) and high-resolution spectroscopy. The extremely large ejection velocity indicates that this star was not ejected by the supernova explosion of a binary companion. Rather, it was probably ejected by a three- or four-body dynamical interaction with more massive objects in a high-density environment. Such a high-density environment may be attained at the core region of a young massive cluster (YMC) with mass of ≳104
                  
                     


                     
                  . The ejection agent that took part in the ejection of LAMOST-HVS1 may be an intermediate mass black hole (≳100 
                     


                     
                  ), a very massive star (≳100 
                     


                     
                  ), or multiple ordinary massive stars (≳30 
                     


                     
                  ). Based on the flight time and the ejection location of LAMOST-HVS1, we argue that its ejection agent or its natal star cluster is currently located near the Norma spiral arm. The natal star cluster of LAMOST-HVS1 may be an undiscovered YMC near the Norma spiral arm.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Hattori, Kohei and Valluri, Monica and Castro, Norberto and Roederer, Ian U. and Mahler, Guillaume and Khullar, Gourav}, year={2019}, month={Mar}, pages={116} }
 @article{mahler_sharon_fox_coe_jauzac_strait_edge_acebron_andrade-santos_avila_et al._2019, title={RELICS: Strong Lensing Analysis of MACS J0417.5–1154 and Predictions for Observing the Magnified High-redshift Universe with <i>JWST}, volume={873}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/ab042b}, DOI={10.3847/1538-4357/ab042b}, abstractNote={Abstract
               Strong gravitational lensing by clusters of galaxies probes the mass distribution at the core of each cluster and magnifies the universe behind it. MACS J0417.5−1154 at z = 0.443 is one of the most massive clusters known based on weak lensing, X-ray, and Sunyaev–Zel’dovich analyses. Here we compute a strong lens model of MACS J0417 based on Hubble Space Telescope imaging observations collected, in part, by the Reionization Lensing Cluster Survey (RELICS), and recently reported spectroscopic redshifts from the MUSE instrument on the Very Large Telescope (VLT). We measure an Einstein radius of 
                     


                     
                    at z = 9 and a mass projected within 200 kpc of 
                     


                     
                  
                  M
                  
                     


                     
                  . Using this model, we measure a ratio between the mass attributed to cluster-member galaxy halos and the main cluster halo of order 1:100. We assess the probability to detect magnified high-redshift galaxies in the field of this cluster, both for comparison with RELICS HST results and as a prediction for the James Webb Space Telescope (JWST) Guaranteed Time Observations upcoming for this cluster. Our lensing analysis indicates that this cluster has similar lensing strength to other clusters in the RELICS program. Our lensing analysis predicts a detection of at least a few z ∼ 6–8 galaxies behind this cluster, at odds with a recent analysis that yielded no such candidates in this field. Reliable strong lensing models are crucial for accurately predicting the intrinsic properties of lensed galaxies. As part of the RELICS program, our strong lensing model produced with the Lenstool parametric method is publicly available through the Mikulski Archive for Space Telescopes.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Mahler, Guillaume and Sharon, Keren and Fox, Carter and Coe, Dan and Jauzac, Mathilde and Strait, Victoria and Edge, Alastair and Acebron, Ana and Andrade-Santos, Felipe and Avila, Roberto J. and et al.}, year={2019}, month={Mar}, pages={96} }
 @article{sakari_roederer_placco_beers_ezzeddine_frebel_hansen_sneden_cowan_wallerstein_et al._2019, title={The <i>R</i>-Process Alliance: Discovery of a Low-<i>α</i>, <i>r</i>-process-enhanced Metal-poor Star in the Galactic Halo}, volume={874}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/ab0c02}, DOI={10.3847/1538-4357/ab0c02}, abstractNote={Abstract
               A new moderately r-process-enhanced metal-poor star, RAVE J093730.5−062655, has been identified in the Milky Way halo as part of an ongoing survey by the R-Process Alliance. The temperature and surface gravity indicate that J0937−0626 is likely a horizontal branch star. At [Fe/H] = −1.86, J0937−0626 is found to have subsolar [X/Fe] ratios for nearly every light, α, and Fe-peak element. The low [α/Fe] ratios can be explained by an ∼0.6 dex excess of Fe; J0937−0626 is therefore similar to the subclass of “iron-enhanced” metal-poor stars. A comparison with Milky Way field stars at [Fe/H] = −2.5 suggests that J0937−0626 was enriched in material from an event, possibly a Type Ia supernova, that created a significant amount of Cr, Mn, Fe, and Ni and smaller amounts of Ca, Sc, Ti, and Zn. The r-process enhancement of J0937−0626 is likely due to a separate event, which suggests that its birth environment was highly enriched in r-process elements. The kinematics of J0937−0626, based on Gaia DR2 data, indicate a retrograde orbit in the Milky Way halo; J0937−0626 was therefore likely accreted from a dwarf galaxy that had significant r-process enrichment.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Sakari, Charli M. and Roederer, Ian U. and Placco, Vinicius M. and Beers, Timothy C. and Ezzeddine, Rana and Frebel, Anna and Hansen, Terese and Sneden, Christopher and Cowan, John J. and Wallerstein, George and et al.}, year={2019}, month={Apr}, pages={148} }
 @article{roederer_buzasi_ji_mace_placco_sobeck_2019, title={The First Stars and the Origin of the Elements}, volume={51}, number={3163}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U. and Buzasi, D. and Ji, A.P. and Mace, G. and Placco, V.M. and Sobeck, J.S.}, year={2019}, pages={163} }
 @article{roederer_2019, title={The Potential of Ultraviolet Spectroscopy To Open New Frontiers To Study the First Stars}, volume={51}, number={3}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2019}, pages={49} }
 @article{placco_santucci_beers_chanamé_sepúlveda_coronado_rossi_lee_starkenburg_youakim_et al._2019, title={The R-Process Alliance: Spectroscopic Follow-up of Low-metallicity Star Candidates from the Best & Brightest Survey}, volume={870}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aaf3b9}, DOI={10.3847/1538-4357/aaf3b9}, abstractNote={Abstract
               We present results from an observing campaign to identify low-metallicity stars in the Best & Brightest Survey. From medium-resolution (R ∼ 1200–2000) spectroscopy of 857 candidates, we estimate the stellar atmospheric parameters (
                     


                     
                  , 
                     


                     
                  , and 
                     


                     
                  ), as well as carbon and α-element abundances. We find that 69% of the observed stars have 
                     


                     
                   ≤ −1.0, 39% have 
                     


                     
                   ≤ −2.0, and 2% have 
                     


                     
                   ≤ −3.0. There are also 133 carbon-enhanced metal-poor (CEMP) stars in this sample, with 97 CEMP Group I and 36 CEMP Group II stars identified in the A(C) versus [Fe/H] diagram. A subset of the confirmed low-metallicity stars were followed-up with high-resolution spectroscopy, as part of the R-process Alliance, with the goal of identifying new highly and moderately r-process-enhanced stars. Comparison between the stellar atmospheric parameters estimated in this work and from high-resolution spectroscopy exhibit good agreement, confirming our expectation that medium-resolution observing campaigns are an effective way of selecting interesting stars for further, more targeted, efforts.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Placco, Vinicius M. and Santucci, Rafael M. and Beers, Timothy C. and Chanamé, Julio and Sepúlveda, María Paz and Coronado, Johanna and Rossi, Silvia and Lee, Young Sun and Starkenburg, Else and Youakim, Kris and et al.}, year={2019}, month={Jan}, pages={122} }
 @article{roederer_beers_rana_frebel_ji_hansen_placco_sakari_2019, title={The astrophysical r-process and the origin of the heaviest elements}, volume={51}, number={3136}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U. and Beers, T.C. and Rana, E. and Frebel, A. and Ji, A.P. and Hansen, T.T. and Placco, V.M. and Sakari, C.M.}, year={2019}, pages={136} }
 @article{roederer_hattori_valluri_2019, title={Using Gaia DR2 to study the kinematics of highly r-process-enhanced stars}, volume={51}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U. and Hattori, K. and Valluri, M.}, year={2019}, pages={23104} }
 @article{horowitz_arcones_côté_dillmann_nazarewicz_roederer_schatz_aprahamian_atanasov_bauswein_et al._2019, title={r</i>-process nucleosynthesis: connecting rare-isotope beam facilities with the cosmos}, volume={46}, ISSN={0954-3899 1361-6471}, url={http://dx.doi.org/10.1088/1361-6471/ab0849}, DOI={10.1088/1361-6471/ab0849}, abstractNote={This is an exciting time for the study of r-process nucleosynthesis. Recently, a neutron star merger GW170817 was observed in extraordinary detail with gravitational waves and electromagnetic radiation from radio to gamma rays. The very red color of the associated kilonova suggests that neutron star mergers are an important r-process site. Astrophysical simulations of neutron star mergers and core collapse supernovae are making rapid progress. Detection of both, electron neutrinos and antineutrinos from the next galactic supernova will constrain the composition of neutrino-driven winds and provide unique nucleosynthesis information. Finally FRIB and other rare-isotope beam facilities will soon have dramatic new capabilities to synthesize many neutron-rich nuclei that are involved in the r-process. The new capabilities can significantly improve our understanding of the r-process and likely resolve one of the main outstanding problems in classical nuclear astrophysics. However, to make best use of the new experimental capabilities and to fully interpret the results, a great deal of infrastructure is needed in many related areas of astrophysics, astronomy, and nuclear theory. We will place these experiments in context by discussing astrophysical simulations and observations of r-process sites, observations of stellar abundances, galactic chemical evolution, and nuclear theory for the structure and reactions of very neutron-rich nuclei. This review paper was initiated at a three-week International Collaborations in Nuclear Theory program in June 2016 where we explored promising r-process experiments and discussed their likely impact, and their astrophysical, astronomical, and nuclear theory context.}, number={8}, journal={Journal of Physics G: Nuclear and Particle Physics}, publisher={IOP Publishing}, author={Horowitz, C J and Arcones, A and Côté, B and Dillmann, I and Nazarewicz, W and Roederer, I U and Schatz, H and Aprahamian, A and Atanasov, D and Bauswein, A and et al.}, year={2019}, month={Jul}, pages={083001} }
 @article{roederer_barklem_2018, title={A New Test of Copper and Zinc Abundances in Late-type Stars Using Ultraviolet Cu ii and Zn ii Lines}, volume={857}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aab71f}, DOI={10.3847/1538-4357/aab71f}, abstractNote={Abstract
               We present new abundances derived from Cu i, Cu ii, Zn i, and Zn ii lines in six warm (5766 ≤ 
                     


                     
                   ≤ 6427 K), metal-poor (−2.50 ≤ [Fe/H] ≤ −0.95) dwarf and subgiant (3.64 ≤ log g ≤ 4.44) stars. These abundances are derived from archival high-resolution ultraviolet spectra from the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope and ground-based optical spectra from several observatories. Ionized Cu and Zn are the majority species, and abundances derived from Cu ii and Zn ii lines should be largely insensitive to departures from local thermodynamic equilibrium (LTE). We find good agreement between the [Zn/H] ratios derived separately from Zn i and Zn ii lines, suggesting that departures from LTE are, at most, minimal (≲0.1 dex). We find that the [Cu/H] ratios derived from Cu ii lines are 0.36 ± 0.06 dex larger than those derived from Cu i lines in the most metal-poor stars ([Fe/H] < −1.8), suggesting that LTE underestimates the Cu abundance derived from Cu i lines. The deviations decrease in more metal-rich stars. Our results validate previous theoretical non-LTE calculations for both Cu and Zn, supporting earlier conclusions that the enhancement of [Zn/Fe] in metal-poor stars is legitimate, and the deficiency of [Cu/Fe] in metal-poor stars may not be as large as previously thought.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Barklem, Paul S.}, year={2018}, month={Apr}, pages={2} }
 @article{roederer_sneden_lawler_sobeck_cowan_boesgaard_2018, title={Consistent Iron Abundances Derived from Neutral and Singly Ionized Iron Lines in Ultraviolet and Optical Spectra of Six Warm Metal-poor Stars}, volume={860}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aac6df}, DOI={10.3847/1538-4357/aac6df}, abstractNote={Abstract
               Neutral Fe lines in metal-poor stars yield conflicting abundances depending on whether and how deviations from local thermodynamic equilibrium (LTE) are considered. We have collected new high-resolution and high signal-to-noise ultraviolet (UV) spectra of three warm dwarf stars with [Fe/H] ≈ −2.9 with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. We locate archival UV spectra for three other warm dwarfs with [Fe/H] ≈ −3.3, −2.2, and −1.6, supplemented with optical spectra for all six stars. We calculate stellar parameters using methods that are largely independent of the spectra, adopting broadband photometry, color–temperature relations, Gaia parallaxes, and assumed masses. We use the LTE line analysis code MOOG to derive Fe abundances from hundreds of Fe i and Fe ii lines with wavelengths from 2290 to 6430 Å. The [Fe/H] ratios derived separately from Fe i and Fe ii lines agree in all six stars, with [Fe ii/H]–[Fe i/H] ranging from +0.00 ± 0.07 to −0.12 ± 0.09 dex, when strong lines and Fe i lines with lower excitation potential <1.2 eV are excluded. This constrains the extent of any deviations from LTE that may occur within this parameter range. While our result confirms non-LTE calculations for some warm, metal-poor dwarfs, it may not be generalizable to more metal-poor dwarfs, where deviations from LTE are predicted to be larger. We also investigate trends of systematically lower abundances derived from Fe i lines in the Balmer continuum region (≈3100–3700 Å), and we conclude that no proposed explanation for this effect can fully account for the observations presently available.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Sneden, Christopher and Lawler, James E. and Sobeck, Jennifer S. and Cowan, John J. and Boesgaard, Ann Merchant}, year={2018}, month={Jun}, pages={125} }
 @inproceedings{aprahamian_surman_frebel_mclaughlin_arcones_balantekin_barnes_beers_holmbeck_yoon_et al._2018, title={FRIB and the GW170817 Kilonova}, DOI={10.48550/arXiv.1809.00703}, abstractNote={In July 2018 an FRIB Theory Alliance program was held on the implications of GW170817 and its associated kilonova for r-process nucleosynthesis. Topics of discussion included the astrophysical and nuclear physics uncertainties in the interpretation of the GW170817 kilonova, what we can learn about the astrophysical site or sites of the r process from this event, and the advances in nuclear experiment and theory most crucial to pursue in light of the new data. Here we compile a selection of scientific contributions to the workshop, broadly representative of progress in r-process studies since the GW170817 event.}, number={1809.007031809.00703}, booktitle={Proceedings for the FRIB Theory Alliance workshop}, author={Aprahamian, A. and Surman, R. and Frebel, A. and McLaughlin, G.C. and Arcones, A. and Balantekin, A.B. and Barnes, J. and Beers, T.C. and Holmbeck, E.M. and Yoon, J. and et al.}, year={2018} }
 @article{roederer_2018, title={Heavy elements in ultra-faint dwarf galaxies}, volume={50}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2018}, pages={232} }
 @article{roederer_hattori_valluri_2018, title={Kinematics of Highly <i>r</i>-process-enhanced Field Stars: Evidence for an Accretion Origin and Detection of Several Groups from Disrupted Satellites}, volume={156}, ISSN={1538-3881}, url={http://dx.doi.org/10.3847/1538-3881/aadd9c}, DOI={10.3847/1538-3881/aadd9c}, abstractNote={Abstract
               We present the kinematics of 35 highly r-process-enhanced ([Eu/Fe] ≥ +0.7) metal-poor (−3.8 < [Fe/H] < −1.4) field stars. We calculate six-dimensional positions and velocities, evaluate energies and integrals of motion, and compute orbits for each of these stars using parallaxes and proper motions from the second Gaia data release and published radial velocities. All of these stars have halo kinematics. Most stars (66%) remain in the inner regions of the halo (<13 kpc), and many (51%) have orbits that pass within 2.6 kpc of the Galactic center. Several stars (20%) have orbits that extend beyond 20 kpc, including one with an orbital apocenter larger than the Milky Way virial radius. We apply three clustering methods to search for structure in phase space, and we identify eight groups. No abundances are considered in the clustering process, but the [Fe/H] dispersions of the groups are smaller than would be expected by random chance. The orbital properties, clustering in phase space and metallicity, and the lack of highly r-process-enhanced stars on disk-like orbits, indicate that such stars likely were accreted from disrupted satellites. Comparison with the galaxy luminosity–metallicity relation suggests M
                  
                     V
                   ≳ −9 for most of the progenitor satellites, characteristic of ultra-faint or low-luminosity classical dwarf spheroidal galaxies. Environments with low rates of star formation and Fe production, rather than the nature of the r-process site, may be key to obtaining the [Eu/Fe] ratios found in highly r-process-enhanced stars.}, number={4}, journal={The Astronomical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Hattori, Kohei and Valluri, Monica}, year={2018}, month={Oct}, pages={179} }
 @article{hattori_valluri_bell_roederer_2018, title={Old, Metal-poor Extreme Velocity Stars in the Solar Neighborhood}, volume={866}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aadee5}, DOI={10.3847/1538-4357/aadee5}, abstractNote={Abstract
               We report the discovery of 30 stars with extreme space velocities (≳480 
                     


                     
                  ) in the Gaia-DR2 archive. These stars are a subset of 1743 stars with high-precision parallax, large tangential velocity (v
                  tan > 300 
                     


                     
                  ), and measured line-of-sight velocity in DR2. By tracing the orbits of the stars back in time, we find at least one of them is consistent with having been ejected by the supermassive black hole at the Galactic Center. Another star has an orbit that passed near the Large Magellanic Cloud about 200 Myr ago. Unlike previously discovered blue hypervelocity stars, our sample is metal-poor (−1.5 < [Fe/H]  < −1.0) and quite old (>1 
                     


                     
                  ). We discuss possible mechanisms for accelerating old stars to such extreme velocities. The high observed space density of this population, relative to potential acceleration mechanisms, implies that these stars are probably bound to the Milky Way (MW). If they are bound, the discovery of this population would require a local escape speed of around ∼600 
                     


                     
                   and consequently imply a virial mass of M
                  200 ∼ 1.4 × 1012
                  M
                  ⊙ for the MW.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Hattori, Kohei and Valluri, Monica and Bell, Eric F. and Roederer, Ian U.}, year={2018}, month={Oct}, pages={121} }
 @article{placco_beers_santucci_chanamé_sepúlveda_coronado_points_kaleida_rossi_kordopatis_et al._2018, title={Spectroscopic Validation of Low-metallicity Stars from RAVE}, volume={155}, ISSN={1538-3881}, url={http://dx.doi.org/10.3847/1538-3881/aac20c}, DOI={10.3847/1538-3881/aac20c}, abstractNote={Abstract
               We present results from a medium-resolution (R ∼ 2000) spectroscopic follow-up campaign of 1694 bright (V < 13.5), very metal-poor star candidates from the RAdial Velocity Experiment (RAVE). Initial selection of the low-metallicity targets was based on the stellar parameters published in RAVE Data Releases 4 and 5. Follow up was accomplished with the Gemini-N and Gemini-S, the ESO/NTT, the KPNO/Mayall, and the SOAR telescopes. The wavelength coverage for most of the observed spectra allows for the determination of carbon and α-element abundances, which are crucial for considering the nature and frequency of the carbon-enhanced metal-poor (CEMP) stars in this sample. We find that 88% of the observed stars have 
                     


                     
                   ≤ −1.0, 61% have 
                     


                     
                   ≤ −2.0, and 3% have 
                     


                     
                   ≤ −3.0 (with four stars at 
                     


                     
                   ≤ −3.5). There are 306 CEMP star candidates in this sample, and we identify 169 CEMP Group I, 131 CEMP Group II, and 6 CEMP Group III stars from the A(C) versus [Fe/H] diagram. Inspection of the 
                     


                     
                   abundance ratios reveals that five of the CEMP Group II stars can be classified as “mono-enriched second-generation” stars. Gaia DR1 matches were found for 734 stars, and we show that transverse velocities can be used as a confirmatory selection criteria for low-metallicity candidates. Selected stars from our validated list are being followed-up with high-resolution spectroscopy to reveal their full chemical-abundance patterns for further studies.}, number={6}, journal={The Astronomical Journal}, publisher={American Astronomical Society}, author={Placco, Vinicius M. and Beers, Timothy C. and Santucci, Rafael M. and Chanamé, Julio and Sepúlveda, María Paz and Coronado, Johanna and Points, Sean D. and Kaleida, Catherine C. and Rossi, Silvia and Kordopatis, Georges and et al.}, year={2018}, month={May}, pages={256} }
 @article{roederer_sakari_placco_beers_ezzeddine_frebel_hansen_2018, title={The <i>R</i>-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-poor Star with an Extreme <i>R</i> -process Enhancement of [Eu/H] = −0.14}, volume={865}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aadd92}, DOI={10.3847/1538-4357/aadd92}, abstractNote={Abstract
               We present a detailed abundance analysis of the bright (V = 9.02), metal-poor ([Fe/H] = −1.47 ± 0.08) field red horizontal-branch star HD 222925, which was observed as part of an ongoing survey by the R-Process Alliance. We calculate stellar parameters and derive abundances for 46 elements based on 901 lines examined in a high-resolution optical spectrum obtained using the Magellan Inamori Kyocera Echelle spectrograph. We detect 28 elements with 38 ≤ Z ≤ 90; their abundance pattern is a close match to the solar r-process component. The distinguishing characteristic of HD 222925 is an extreme enhancement of r-process elements ([Eu/Fe] = +1.33 ± 0.08, [Ba/Eu] = −0.78 ± 0.10) in a moderately metal-poor star, so the abundance of r-process elements is the highest ([Eu/H] = −0.14 ± 0.09) in any known r-process-enhanced star. The abundance ratios among lighter (Z ≤ 30) elements are typical for metal-poor stars, indicating that production of these elements was dominated by normal Type II supernovae, with no discernible contributions from Type Ia supernovae or asymptotic giant branch stars. The chemical and kinematic properties of HD 222925 suggest it formed in a low-mass dwarf galaxy, which was enriched by a high-yield r-process event before being disrupted by interaction with the Milky Way.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Sakari, Charli M. and Placco, Vinicius M. and Beers, Timothy C. and Ezzeddine, Rana and Frebel, Anna and Hansen, Terese T.}, year={2018}, month={Oct}, pages={129} }
 @article{cain_frebel_gull_ji_placco_beers_meléndez_ezzeddine_casey_hansen_et al._2018, title={The <i>R</i>-Process Alliance: Chemical Abundances for a Trio of <i>r</i>-process-enhanced Stars—One Strong, One Moderate, and One Mild}, volume={864}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aad37d}, DOI={10.3847/1538-4357/aad37d}, abstractNote={Abstract
               We present detailed chemical abundances of three new bright (V ∼ 11), extremely metal-poor ([Fe/H] ∼ −3.0), r-process-enhanced halo red giants based on high-resolution, high-S/N Magellan/MIKE spectra. We measured abundances for 20–25 neutron-capture elements in each of our stars. J1432−4125 is among the most r-process-rich r-II stars, with [Eu/Fe] = +1.44 ± 0.11. J2005−3057 is an r-I star with [Eu/Fe] = +0.94 ± 0.07. J0858−0809 has [Eu/Fe] = +0.23 ± 0.05 and exhibits a carbon abundance corrected for an evolutionary status of [C/Fe]corr = +0.76, thus adding to the small number of known carbon-enhanced r-process stars. All three stars show remarkable agreement with the scaled solar r-process pattern for elements above Ba, consistent with enrichment of the birth gas cloud by a neutron star merger. The abundances for Sr, Y, and Zr, however, deviate from the scaled solar pattern. This indicates that more than one distinct r-process site might be responsible for the observed neutron-capture element abundance pattern. Thorium was detected in J1432−4125 and J2005−3057. Age estimates for J1432−4125 and J2005−3057 were adopted from one of two sets of initial production ratios each by assuming the stars are old. This yielded individual ages of 12 ± 6 Gyr and 10 ± 6 Gyr, respectively.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Cain, Madelyn and Frebel, Anna and Gull, Maude and Ji, Alexander P. and Placco, Vinicius M. and Beers, Timothy C. and Meléndez, Jorge and Ezzeddine, Rana and Casey, Andrew R. and Hansen, Terese T. and et al.}, year={2018}, month={Aug}, pages={43} }
 @article{sakari_placco_farrell_roederer_wallerstein_beers_ezzeddine_frebel_hansen_holmbeck_et al._2018, title={The <i>R</i>-Process Alliance: First Release from the Northern Search for <i>r</i>-process-enhanced Metal-poor Stars in the Galactic Halo}, volume={868}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aae9df}, DOI={10.3847/1538-4357/aae9df}, abstractNote={Abstract
               This paper presents the detailed abundances and r-process classifications of 126 newly identified metal-poor stars as part of an ongoing collaboration, the R-Process Alliance. The stars were identified as metal-poor candidates from the RAdial Velocity Experiment (RAVE) and were followed up at high spectral resolution (R ∼ 31,500) with the 3.5 m telescope at Apache Point Observatory. The atmospheric parameters were determined spectroscopically from Fe i lines, taking into account 
                     


                     
                   non-LTE corrections and using differential abundances with respect to a set of standards. Of the 126 new stars, 124 have [Fe/H] < −1.5, 105 have [Fe/H] < −2.0, and 4 have [Fe/H] < −3.0. Nine new carbon-enhanced metal-poor stars have been discovered, three of which are enhanced in r-process elements. Abundances of neutron-capture elements reveal 60 new r-I stars (with +0.3 ≤ [Eu/Fe] ≤ +1.0 and [Ba/Eu] < 0) and 4 new r-II stars (with [Eu/Fe] > +1.0). Nineteen stars are found to exhibit a “limited-r” signature ([Sr/Ba] > +0.5, [Ba/Eu] < 0). For the r-II stars, the second- and third-peak main r-process patterns are consistent with the r-process signature in other metal-poor stars and the Sun. The abundances of the light, α, and Fe-peak elements match those of typical Milky Way (MW) halo stars, except for one r-I star that has high Na and low Mg, characteristic of globular cluster stars. Parallaxes and proper motions from the second Gaia data release yield UVW space velocities for these stars that are consistent with membership in the MW halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Sakari, Charli M. and Placco, Vinicius M. and Farrell, Elizabeth M. and Roederer, Ian U. and Wallerstein, George and Beers, Timothy C. and Ezzeddine, Rana and Frebel, Anna and Hansen, Terese and Holmbeck, Erika M. and et al.}, year={2018}, month={Nov}, pages={110} }
 @article{hansen_holmbeck_beers_placco_roederer_frebel_sakari_simon_thompson_2018, title={The <i>R</i>-process Alliance: First Release from the Southern Search for <i>R</i>-process-enhanced Stars in the Galactic Halo}, volume={858}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aabacc}, DOI={10.3847/1538-4357/aabacc}, abstractNote={Abstract
               The recent detection of a binary neutron star merger and the clear evidence of the decay of radioactive material observed in this event have, after 60 years of effort, provided an astrophysical site for the rapid neutron-capture (r-) process which is responsible for the production of the heaviest elements in our universe. However, observations of metal-poor stars with highly enhanced r-process elements have revealed abundance patterns suggesting that multiple sites may be involved. To address this issue, and to advance our understanding of the r-process, we have initiated an extensive search for bright (V < 13.5), very metal-poor ([Fe/H] < −2) stars in the Milky Way halo exhibiting strongly enhanced r-process signatures. This paper presents the first sample collected in the southern hemisphere using the echelle spectrograph on du Pont 2.5 m telescope at Las Campanas Observatory. We have observed and analyzed 107 stars with −3.13 < [Fe/H] < −0.79. Of those, 12 stars are strongly enhanced in heavy r-process elements (r-II), 42 stars show moderate enhancements of heavy r-process material (r-I), and 20 stars exhibit low abundances of the heavy r-process elements and higher abundances of the light r-process elements relative to the heavy ones (limited-r). This search is more successful at finding r-process-enhanced stars compared to previous searches, primarily due to a refined target selection procedure that focuses on red giants.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Hansen, Terese T. and Holmbeck, Erika M. and Beers, Timothy C. and Placco, Vinicius M. and Roederer, Ian U. and Frebel, Anna and Sakari, Charli M. and Simon, Joshua D. and Thompson, Ian B.}, year={2018}, month={May}, pages={92} }
 @article{holmbeck_beers_roederer_placco_hansen_sakari_sneden_liu_lee_cowan_et al._2018, title={The R-Process Alliance: 2MASS J09544277+5246414, the Most Actinide-enhanced R-II Star Known}, url={https://doi.org/10.3847/2041-8213/aac722}, DOI={10.3847/2041-8213/aac722}, abstractNote={Abstract
               We report the discovery of a new actinide-boost star, 2MASS J09544277+5246414, originally identified as a very bright (V = 10.1), extremely metal-poor ([Fe/H] = −2.99) K giant in the LAMOST survey, and found to be highly r-process-enhanced (r-II; [Eu/Fe] = +1.28]), during the snapshot phase of the R-Process Alliance (RPA). Based on a high signal-to-noise ratio (S/N), high-resolution spectrum obtained with the Harlan J. Smith 2.7 m telescope, this star is the first confirmed actinide-boost star found by RPA efforts. With an enhancement of [Th/Eu] = +0.37, 2MASS J09544277+5246414 is also the most actinide-enhanced r-II star yet discovered, and only the sixth metal-poor star with a measured uranium abundance ([U/Fe] = +1.40). Using the Th/U chronometer, we estimate an age of 13.0 ± 4.7 Gyr for this star. The unambiguous actinide-boost signature of this extremely metal-poor star, combined with additional r-process-enhanced and actinide-boost stars identified by the RPA, will provide strong constraints on the nature and origin of the r-process at early times.}, journal={The Astrophysical Journal Letters}, author={Holmbeck, Erika M. and Beers, Timothy C. and Roederer, Ian U. and Placco, Vinicius M. and Hansen, Terese T. and Sakari, Charli M. and Sneden, Christopher and Liu, Chao and Lee, Young Sun and Cowan, John J. and et al.}, year={2018}, month={Jun} }
 @article{sakari_placco_hansen_holmbeck_beers_frebel_roederer_venn_wallerstein_davis_et al._2018, title={The r-process Pattern of a Bright, Highly r-process-enhanced Metal-poor Halo Star at [Fe/H] ∼ −2}, url={https://doi.org/10.3847/2041-8213/aaa9b4}, DOI={10.3847/2041-8213/aaa9b4}, abstractNote={Abstract
               A high-resolution spectroscopic analysis is presented for a new highly r-process-enhanced ([Eu/Fe] = 1.27, [Ba/Eu] = −0.65), very metal-poor ([Fe/H] = −2.09), retrograde halo star, RAVE J153830.9–180424, discovered as part of the R-Process Alliance survey. At V = 10.86, this is the brightest and most metal-rich r-II star known in the Milky Way halo. Its brightness enables high-S/N detections of a wide variety of chemical species that are mostly created by the r-process, including some infrequently detected lines from elements like Ru, Pd, Ag, Tm, Yb, Lu, Hf, and Th, with upper limits on Pb and U. This is the most complete r-process census in a very metal-poor r-II star. J1538–1804 shows no signs of s-process contamination, based on its low [Ba/Eu] and [Pb/Fe]. As with many other r-process-enhanced stars, J1538–1804's r-process pattern matches that of the Sun for elements between the first, second, and third peaks, and does not exhibit an actinide boost. Cosmo-chronometric age-dating reveals the r-process material to be quite old. This robust main r-process pattern is a necessary constraint for r-process formation scenarios (of particular interest in light of the recent neutron star merger, GW170817), and has important consequences for the origins of r-II stars. Additional r-I and r-II stars will be reported by the R-Process Alliance in the near future.}, journal={The Astrophysical Journal Letters}, author={Sakari, Charli M. and Placco, Vinicius M. and Hansen, Terese and Holmbeck, Erika M. and Beers, Timothy C. and Frebel, Anna and Roederer, Ian U. and Venn, Kim A. and Wallerstein, George and Davis, Christopher Evan and et al.}, year={2018}, month={Feb} }
 @article{roederer_2018, title={Using CETUS to study the first stars and first metals}, volume={50}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2018}, pages={140.14} }
 @article{roederer_2018, title={Using r-process enhanced galaxies to estimate the neutron star merger rate at high redshift}, volume={50}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2018}, pages={231} }
 @article{song_mateo_walker_roederer_2017, title={An Expanded Chemo-dynamical Sample of Red Giants in the Bar of the Large Magellanic Cloud}, volume={153}, ISSN={1538-3881}, url={http://dx.doi.org/10.3847/1538-3881/aa6eaa}, DOI={10.3847/1538-3881/aa6eaa}, abstractNote={Abstract
               We report new spectroscopic observations obtained with the Michigan/Magellan Fiber System of 308 red giants (RGs) located in two fields near the photometric center of the bar of the Large Magellanic Cloud. This sample consists of 131 stars observed in previous studies (in one field) and 177 newly observed stars (in the second field) selected specifically to more reliably establish the metallicity and age distributions of the bar. For each star, we measure its heliocentric line-of-sight velocity, surface gravity, and metallicity from its high-resolution spectrum (effective temperatures come from photometric colors). The spectroscopic Hertzsprung–Russell diagrams—modulo small offsets in surface gravities—reveal good agreement with model isochrones. The mean metallicity of the 177-RG sample is [Fe/H] = −0.76 ± 0.02 with a metallicity dispersion σ = 0.28 ± 0.03. The corresponding metallicity distribution—corrected for selection effects—is well fitted by two Gaussian components: one metal-rich with a mean −0.66 ± 0.02 and a standard deviation 0.17 ± 0.01, and the other metal-poor with −1.20 ± 0.24 and 0.41 ± 0.06. The metal-rich and metal-poor populations contain approximately 85% and 15% of stars, respectively. We also confirm that the velocity dispersion in the bar center decreases significantly from 31.2 ± 4.3 to 18.7 ± 1.9 km s−1 with increasing metallicity over the range −2.09 to −0.38. Individual stellar masses are estimated using the spectroscopic surface gravities and the known luminosities. We find that lower mass, hence older, RGs have larger metallicity dispersion and lower mean metallicity than the higher-mass, younger RGs. The estimated masses, however, extend to implausibly low values (
                     


                     
                  ), making it impossible to obtain an absolute age–metallicity or age distribution of the bar.}, number={6}, journal={The Astronomical Journal}, publisher={American Astronomical Society}, author={Song, Ying-Yi and Mateo, Mario and Walker, Matthew G. and Roederer, Ian U.}, year={2017}, month={May}, pages={261} }
 @inproceedings{roederer_2017, title={An update on the r-process one year after the discovery of the Reticulum II dwarf galaxy}, booktitle={2017 JINA-CEE Frontiers in Nuclear Astrophysics}, author={Roederer, I.U.}, year={2017} }
 @article{scowen_tripp_beasley_ardila_andersson_apellániz_barstow_bianchi_calzetti_clampin_et al._2017, title={Finding the UV–Visible Path Forward: Proceedings of the Community Workshop to Plan the Future of UV/Visible Space Astrophysics}, volume={129}, ISSN={0004-6280 1538-3873}, url={http://dx.doi.org/10.1088/1538-3873/129/977/076001}, DOI={10.1088/1538-3873/129/977/076001}, abstractNote={We present the science cases and technological discussions that came from the workshop titled “Finding the ultraviolet (UV)-Visible Path Forward” held at NASA GSFC 2015 June 25–26. The material presented outlines the compelling science that can be enabled by a next generation space-based observatory dedicated for UV–visible science, the technologies that are available to include in that observatory design, and the range of possible alternative launch approaches that could also enable some of the science. The recommendations to the Cosmic Origins Program Analysis Group from the workshop attendees on possible future development directions are outlined.}, number={977}, journal={Publications of the Astronomical Society of the Pacific}, publisher={IOP Publishing}, author={Scowen, Paul A. and Tripp, Todd and Beasley, Matt and Ardila, David and Andersson, B.-G. and Apellániz, Jesús Maíz and Barstow, Martin and Bianchi, Luciana and Calzetti, Daniela and Clampin, Mark and et al.}, year={2017}, month={Jun}, pages={076001} }
 @article{roederer_2017, title={Hubble Space Telescope</i>Ultraviolet Spectroscopy of the Most Iron-poor Star Known}, volume={1}, ISSN={2515-5172}, url={http://dx.doi.org/10.3847/2515-5172/aaa42e}, DOI={10.3847/2515-5172/aaa42e}, number={1}, journal={Research Notes of the AAS}, publisher={American Astronomical Society}, author={Roederer, Ian U.}, year={2017}, month={Dec}, pages={56} }
 @article{sterling_madonna_butler_garcía-rojas_mashburn_morisset_luridiana_roederer_2017, title={Identification of Near-infrared [Se iii] and [Kr vi] Emission Lines in Planetary Nebulae}, volume={840}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/aa6c28}, DOI={10.3847/1538-4357/aa6c28}, abstractNote={Abstract
               We identify [Se iii] 1.0994 μm in the planetary nebula (PN) NGC 5315 and [Kr vi] 1.2330 μm in three PNe from spectra obtained with the Folded-Port InfraRed Echellette (FIRE) spectrometer on the 6.5 m Baade Telescope. Se and Kr are the two most widely detected neutron-capture elements in astrophysical nebulae, and can be enriched by s-process nucleosynthesis in PN progenitor stars. The detection of [Se iii] 1.0994 μm is particularly valuable when paired with observations of [Se iv] 2.2864 μm, as it can be used to improve the accuracy of nebular Se abundance determinations, and allows Se ionization correction factor (ICF) schemes to be empirically tested for the first time. We present new effective collision strength calculations for Se2+ and Kr5+, which we use to compute ionic abundances. In NGC 5315, we find that the Se abundance computed from Se3+/H+ is lower than that determined with ICFs that incorporate Se2+/H+. We compute new Kr ICFs that take Kr5+/H+ into account, by fitting correlations found in grids of Cloudy models between Kr ionic fractions and those of more abundant elements, and use these to derive Kr abundances in four PNe. Observations of [Se iii] and [Kr vi] in a larger sample of PNe, with a range of excitation levels, are needed to rigorously test the ICF prescriptions for Se and our new Kr ICFs.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Sterling, N. C. and Madonna, S. and Butler, K. and García-Rojas, J. and Mashburn, A. L. and Morisset, C. and Luridiana, V. and Roederer, I. U.}, year={2017}, month={May}, pages={80} }
 @article{madonna_garcía-rojas_sterling_delgado-inglada_mesa-delgado_luridiana_roederer_mashburn_2017, title={Neutron-capture element abundances in the planetary nebula NGC 5315 from deep optical and near-infrared spectrophotometry★†}, volume={471}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stx1585}, DOI={10.1093/mnras/stx1585}, abstractNote={We analyze the chemical composition of the planetary nebula (PN) NGC 5315, through high-resolution (R = 40000) optical spectroscopy with UVES at the Very Large Telescope, and medium-resolution (R = 4800) near-infrared spectroscopy with FIRE at Magellan Baade Telescope, covering a wide spectral range from 0.31 to 2.50 micron. The main aim of this work is to investigate neutron (n)-capture element abundances to study the operation of the slow n-capture ("s-process") in the AGB progenitor of NGC 5315. We detect more than 700 emission lines, including ions of the n-capture elements Se, Kr, Xe, and possibly Br. We compute physical conditions from a large number of diagnostic line ratios, and derive ionic abundances for species with available atomic data. The total abundances are computed using recent ionization correction factors (ICFs) or by summing ionic abundances. Total abundances of common elements are in good agreement with previous work on this object. Based on our abundance analysis of NGC 5315, including the lack of s-process enrichment, we speculate that the most probable scenario is that the progenitor star is in a binary system as hinted at by radial velocity studies, and interactions with its companion truncated the AGB before s-process enrichment could occur. However there are other two possible scenarios for its evolution, that cannot be ruled out: i) the progenitor is a low-mass single star that did not undergo third dredge-up; ii) the progenitor star of NGC 5315 had an initial mass of 4--6 solar masses, and any s-process enhancements were heavily diluted by the massive envelope during the AGB phase.}, number={2}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Madonna, S. and García-Rojas, J. and Sterling, N. C. and Delgado-Inglada, G. and Mesa-Delgado, A. and Luridiana, V. and Roederer, I. U. and Mashburn, A. L.}, year={2017}, month={Jun}, pages={1341–1369} }
 @inproceedings{roederer_2017, title={Observations of CEMP-i Stars}, booktitle={A Celebration of CEMP and a Gala of GALAH}, author={Roederer, I.U.}, year={2017} }
 @article{roederer_2017, title={THE ORIGIN OF THE HEAVIEST METALS IN MOST ULTRA-FAINT DWARF GALAXIES}, volume={835}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/1538-4357/835/1/23}, DOI={10.3847/1538-4357/835/1/23}, abstractNote={ABSTRACT
               The heaviest metals found in stars in most ultra-faint dwarf (UFD) galaxies in the Milky Way halo are generally underabundant by an order of magnitude or more when compared with stars in the halo field. Among the heavy elements produced by n-capture reactions, only Sr and Ba can be detected in red giant stars in most UFD galaxies. This limited chemical information is unable to identify the nucleosynthesis process(es) responsible for producing the heavy elements in UFD galaxies. Similar [Sr/Ba] and [Ba/Fe] ratios are found in three bright halo field stars, BD−18°5550, CS 22185–007, and CS 22891–200. Previous studies of high-quality spectra of these stars report detections of additional n-capture elements, including Eu. The [Eu/Ba] ratios in these stars span +0.41 to +0.86. These ratios and others among elements in the rare Earth domain indicate an r-process origin. These stars have some of the lowest levels of r-process enhancement known, with [Eu/H] spanning −3.95 to −3.32, and they may be considered nearby proxies for faint stars in UFD galaxies. Direct confirmation, however, must await future observations of additional heavy elements in stars in the UFD galaxies themselves.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U.}, year={2017}, month={Jan}, pages={23} }
 @inproceedings{roederer_2017, title={The environment of the r-process}, booktitle={Forging Connections: from Nuclei to the Cosmic Web}, author={Roederer, I.U.}, year={2017} }
 @article{roederer_mateo_iii_song_bell_crane_loebman_nidever_olszewski_shectman_et al._2016, title={DETAILED CHEMICAL ABUNDANCES IN THE<i>r</i>-PROCESS-RICH ULTRA-FAINT DWARF GALAXY RETICULUM 2}, volume={151}, ISSN={1538-3881}, url={http://dx.doi.org/10.3847/0004-6256/151/3/82}, DOI={10.3847/0004-6256/151/3/82}, abstractNote={ABSTRACT
               The ultra-faint dwarf (UFD) galaxy Reticulum 2 (Ret 2) was recently discovered in images obtained by the Dark Energy Survey. We have observed the four brightest red giants in Ret 2 at high spectral resolution using the Michigan/Magellan Fiber System. We present detailed abundances for as many as 20 elements per star, including 12 elements heavier than the Fe group. We confirm previous detection of high levels of r-process material in Ret 2 (mean [Eu/Fe] = +1.69 ± 0.05) found in three of these stars (mean [Fe/H] = −2.88 ± 0.10). The abundances closely match the r-process pattern found in the well-studied metal-poor halo star CS 22892–052. Such r-process-enhanced stars have not been found in any other UFD galaxy, though their existence has been predicted by at least one model. The fourth star in Ret 2 ([Fe/H] = −3.42 ± 0.20) contains only trace amounts of Sr ([Sr/Fe] = −1.73 ± 0.43) and no detectable heavier elements. One r-process enhanced star is also enhanced in C (natal [C/Fe] ≈ +1.1). This is only the third such star known, which suggests that the nucleosynthesis sites leading to C and r-process enhancements are decoupled. The r-process-deficient star is enhanced in Mg ([Mg/Fe] = +0.81 ± 0.14), and the other three stars show normal levels of α-enhancement (mean [Mg/Fe] = +0.34 ± 0.03). The abundances of other α and Fe-group elements closely resemble those in UFD galaxies and metal-poor halo stars, suggesting that the nucleosynthesis that led to the large r-process enhancements either produced no light elements or produced light-element abundance signatures indistinguishable from normal supernovae.}, number={3}, journal={The Astronomical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Mateo, Mario and Iii, John I. Bailey and Song, Yingyi and Bell, Eric F. and Crane, Jeffrey D. and Loebman, Sarah and Nidever, David L. and Olszewski, Edward W. and Shectman, Stephen A. and et al.}, year={2016}, month={Feb}, pages={82} }
 @article{roederer_placco_beers_2016, title={DETECTION OF PHOSPHORUS, SULPHUR, AND ZINC IN THE CARBON-ENHANCED METAL-POOR STAR BD+44 493*}, url={https://doi.org/10.3847/2041-8205/824/2/L19}, DOI={10.3847/2041-8205/824/2/L19}, abstractNote={ABSTRACT
               The carbon-enhanced metal-poor star BD+44°493 ([Fe/H] = −3.9) has been proposed as a candidate second-generation star enriched by metals from a single Pop III star. We report the first detections of P and S and the second detection of Zn in any extremely metal-poor carbon-enhanced star, using new spectra of BD+44°493 collected by the Cosmic Origins Spectrograph on the Hubble Space Telescope. We derive [P/Fe] = −0.34 ± 0.21, [S/Fe] = +0.07 ± 0.41, and [Zn/Fe] = −0.10 ± 0.24. We increase by 10-fold the number of Si i lines detected in BD+44°493, yielding [Si/Fe] = +0.15 ± 0.22. The [S/Fe] and [Zn/Fe] ratios exclude the hypothesis that the abundance pattern in BD+44°493 results from depletion of refractory elements onto dust grains. Comparison with zero-metallicity supernova (SN) models suggests that the stellar progenitor that enriched BD+44°493 was massive and ejected much less than 0.07 M
                  ⊙ of 56Ni, characteristic of a faint SN.}, journal={The Astrophysical Journal Letters}, author={Roederer, Ian U. and Placco, Vinicius M. and Beers, Timothy C.}, year={2016}, month={Jun} }
 @inproceedings{roederer_2016, title={Different Ways of Thinking about r-process observations}, booktitle={The r-process nucleosynthesis: connecting FRIB with the cosmos}, author={Roederer, I.U.}, year={2016} }
 @inproceedings{roederer_2016, title={Heavy Metals in Dwarf Galaxies}, booktitle={4th Magellan Science Symposium}, author={Roederer, I.U.}, year={2016} }
 @article{mashburn_sterling_madonna_dinerstein_roederer_geballe_2016, title={NEUTRON-CAPTURE ELEMENT ABUNDANCES IN MAGELLANIC CLOUD PLANETARY NEBULAE*}, url={https://doi.org/10.3847/2041-8205/831/1/L3}, DOI={10.3847/2041-8205/831/1/L3}, abstractNote={ABSTRACT
               We present near-infrared spectra of 10 planetary nebulae (PNe) in the Large and Small Magellanic Clouds (LMC and SMC), acquired with the FIRE and GNIRS spectrometers on the 6.5 m Baade and 8.1 m Gemini South Telescopes, respectively. We detect Se and/or Kr emission lines in eight of these objects, the first detections of n-capture elements in Magellanic Cloud PNe. Our abundance analysis shows large s-process enrichments of Kr (0.6–1.3 dex) in the six PNe in which it was detected, and Se is enriched by 0.5–0.9 dex in five objects. We also estimate upper limits to Rb and Cd abundances in these objects. Our abundance results for the LMC are consistent with the hypothesis that PNe with 2–3 M
                  ⊙ progenitors dominate the bright end of the PN luminosity function in young gas-rich galaxies. We find no significant correlations between s-process enrichments and other elemental abundances, central star temperature, or progenitor mass, though this is likely due to our small sample size. We determine S abundances from our spectra and find that [S/H] agrees with [Ar/H] to within 0.2 dex for most objects, but is lower than [O/H] by 0.2–0.4 dex in some PNe, possibly due to O enrichment via third dredge-up. Our results demonstrate that n-capture elements can be detected in PNe belonging to nearby galaxies with ground-based telescopes, allowing s-process enrichments to be studied in PN populations with well-determined distances.}, journal={The Astrophysical Journal Letters}, author={Mashburn, A. L. and Sterling, N. C. and Madonna, S. and Dinerstein, Harriet L. and Roederer, I. U. and Geballe, T. R.}, year={2016}, month={Nov} }
 @inproceedings{roederer_2016, title={Neutron-Capture Elements from the First Stars}, booktitle={First Stars V}, author={Roederer, I.U.}, year={2016} }
 @inproceedings{roederer_2016, title={New Constraints on the r-process Provided by New Observations of Rare Elements in Metal-Poor Stars}, booktitle={JINA-CEE Frontiers Meeting}, author={Roederer, I.U.}, year={2016} }
 @inproceedings{roederer_2016, title={New Observational Constraints from Ancient Stars on the Origins of Heavy Elements}, booktitle={JINA-CEE Frontiers Meeting}, author={Roederer, I.U.}, year={2016} }
 @article{yoon_beers_placco_rasmussen_carollo_he_hansen_roederer_zeanah_2016, title={OBSERVATIONAL CONSTRAINTS ON FIRST-STAR NUCLEOSYNTHESIS. I. EVIDENCE FOR MULTIPLE PROGENITORS OF CEMP-NO STARS}, volume={833}, ISSN={1538-4357}, url={http://dx.doi.org/10.3847/0004-637x/833/1/20}, DOI={10.3847/0004-637X/833/1/20}, abstractNote={ABSTRACT
               We investigate anew the distribution of absolute carbon abundance, A(C) = log ϵ(C), for carbon-enhanced metal-poor (CEMP) stars in the halo of the Milky Way, based on high-resolution spectroscopic data for a total sample of 305 CEMP stars. The sample includes 147 CEMP-s (and CEMP-r/s) stars, 127 CEMP-no stars, and 31 CEMP stars that are unclassified, based on the currently employed [Ba/Fe] criterion. We confirm previous claims that the distribution of A(C) for CEMP stars is (at least) bimodal, with newly determined peaks centered on A(C) = 7.96 (the high-C region) and A(C) = 6.28 (the low-C region). A very high fraction of CEMP-s (and CEMP-r/s) stars belongs to the high-C region, while the great majority of CEMP-no stars resides in the low-C region. However, there exists complexity in the morphology of the A(C)-[Fe/H] space for the CEMP-no stars, a first indication that more than one class of first-generation stellar progenitors may be required to account for their observed abundances. The two groups of CEMP-no stars we identify exhibit clearly different locations in the A(Na)-A(C) and A(Mg)-A(C) spaces, also suggesting multiple progenitors. The clear distinction in A(C) between the CEMP-s (and CEMP-r/s) stars and the CEMP-no stars appears to be as successful, and likely more astrophysically fundamental, for the separation of these sub-classes as the previously recommended criterion based on [Ba/Fe] (and [Ba/Eu]) abundance ratios. This result opens the window for its application to present and future large-scale low- and medium-resolution spectroscopic surveys.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Yoon, Jinmi and Beers, Timothy C. and Placco, Vinicius M. and Rasmussen, Kaitlin C. and Carollo, Daniela and He, Siyu and Hansen, Terese T. and Roederer, Ian U. and Zeanah, Jeff}, year={2016}, month={Dec}, pages={20} }
 @article{roederer_karakas_pignatari_herwig_2016, title={THE DIVERSE ORIGINS OF NEUTRON-CAPTURE ELEMENTS IN THE METAL-POOR STAR HD 94028: POSSIBLE DETECTION OF PRODUCTS OF i-PROCESS NUCLEOSYNTHESIS*}, url={https://doi.org/10.3847/0004-637X/821/1/37}, DOI={10.3847/0004-637X/821/1/37}, abstractNote={ABSTRACT
               We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H] = −1.62 ± 0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe] = +0.79 ± 0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe] = +0.22 ± 0.12), including unusually large molybdenum ([Mo/Fe] = +0.97 ± 0.16) and ruthenium ([Ru/Fe] = +0.69 ± 0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe] = −0.06 ± 0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99 ± 0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy.}, journal={The Astrophysical Journal}, author={Roederer, Ian U. and Karakas, Amanda I. and Pignatari, Marco and Herwig, Falk}, year={2016}, month={Apr} }
 @article{roederer_thompson_2015, title={Detailed abundances of 15 stars in the metal-poor globular cluster NGC 4833★}, volume={449}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stv546}, DOI={10.1093/mnras/stv546}, abstractNote={We have observed 15 red giant stars in the relatively massive, metal-poor globular cluster NGC 4833 using the Magellan Inamori Kyocera Echelle spectrograph at Magellan. We calculate stellar parameters for each star and perform a standard abundance analysis to derive abundances of 43 species of 39 elements, including 20 elements heavier than the iron group. We derive = -2.25 +/- 0.02 from Fe I lines and = -2.19 +/- 0.013 from Fe II lines. We confirm earlier results that found no internal metallicity spread in NGC 4833, and there are no significant star-to-star abundance dispersions among any elements in the iron group (19 <= Z <= 30). We recover the usual abundance variations among the light elements C, N, O, Na, Mg, Al, and possibly Si. The heavy-element distribution reflects enrichment by r-process nucleosynthesis ([Eu/Fe] = +0.36 +/- 0.03), as found in many other metal-poor globular clusters. We investigate small star-to-star variations found among the neutron-capture elements, and we conclude that these are probably not real variations. Upper limits on the Th abundance, log epsilon (Th/Eu) < -0.47 +/- 0.09, indicate that NGC 4833, like other globular clusters where Th has been studied, did not experience a so-called "actinide boost."}, number={4}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Roederer, Ian U. and Thompson, Ian B.}, year={2015}, month={Apr}, pages={3889–3910} }
 @article{roederer_mateo_bailey_spencer_crane_shectman_2015, title={Detailed chemical abundances in NGC 5824: another metal-poor globular cluster with internal heavy element abundance variations}, volume={455}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stv2462}, DOI={10.1093/mnras/stv2462}, abstractNote={We present radial velocities, stellar parameters, and detailed abundances of 39 elements derived from high-resolution spectroscopic observations of red giant stars in the luminous, metal-poor globular cluster NGC 5824. We observe 26 stars in NGC 5824 using the Michigan/Magellan Fiber System (M2FS) and two stars using the Magellan Inamori Kyocera Echelle (MIKE) spectrograph. We derive a mean metallicity of [Fe/H]=-1.94+/-0.02 (statistical) +/-0.10 (systematic). The metallicity dispersion of this sample of stars, 0.08 dex, is in agreement with previous work and does not exceed the expected observational errors. Previous work suggested an internal metallicity spread only when fainter samples of stars were considered, so we cannot exclude the possibility of an intrinsic metallicity dispersion in NGC 5824. The M2FS spectra reveal a large internal dispersion in [Mg/Fe], 0.28 dex, which is found in a few other luminous, metal-poor clusters. [Mg/Fe] is correlated with [O/Fe] and anti-correlated with [Na/Fe] and [Al/Fe]. There is no evidence for internal dispersion among the other alpha- or Fe-group abundance ratios. Twenty-five of the 26 stars exhibit a n-capture enrichment pattern dominated by r-process nucleosynthesis ([Eu/Fe]=+0.11+/-0.12; [Ba/Eu]=-0.66+/-0.05). Only one star shows evidence of substantial s-process enhancement ([Ba/Fe]=+0.56+/-0.12; [Ba/Eu]=+0.38+/-0.14), but this star does not exhibit other characteristics associated with s-process enhancement via mass-transfer from a binary companion. The Pb and other heavy elements produced by the s-process suggest a timescale of no more than a few hundred Myr for star formation and chemical enrichment, like the complex globular clusters M2, M22, and NGC 5286.}, number={3}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Roederer, Ian U. and Mateo, Mario and Bailey, John I., III and Spencer, Meghin and Crane, Jeffrey D. and Shectman, Stephen A.}, year={2015}, month={Nov}, pages={2417–2439} }
 @article{placco_beers_ivans_filler_imig_roederer_abate_hansen_cowan_frebel_et al._2015, title={HUBBLE SPACE TELESCOPE NEAR-ULTRAVIOLET SPECTROSCOPY OF BRIGHT CEMP-s STARS}, volume={812}, DOI={10.1088/0004-637X/812/2/109}, abstractNote={We present an elemental-abundance analysis, in the near-ultraviolet (NUV) spectral range, for the bright carbon-enhanced metal-poor (CEMP) stars HD 196944 ( V = 8.40 , ?> [Fe/H] = −2.41) and HD 201626 ( V = 8.16 , ?> [Fe/H] = −1.51), based on data acquired with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope. Both of these stars belong to the sub-class CEMP-s, and exhibit clear over-abundances of heavy elements associated with production by the slow neutron-capture process. HD 196944 has been well-studied in the optical region, but we add abundance results for six species (Ge, Nb, Mo, Lu, Pt, and Au) that are only accessible in the NUV. In addition, we provide the first determination of its orbital period, P = 1325 days. HD 201626 has only a limited number of abundance results based on previous optical work—here we add five new species from the NUV, including Pb. We compare these results with models of binary-system evolution and s-process element production in stars on the asymptotic giant branch, with the goal of explaining their origin and evolution. Our best-fitting models for HD 196944 ( M 1 , i = 0.9 M ⊙ , ?> M 2 , i = 0.86 M ⊙ , ?> for [Fe/H] = − 2.2 ?> ), and HD 201626 ( M 1 , i = 0.9 M ⊙ , ?> M 2 , i = 0.76 M ⊙ , ?> for [Fe/H] = − 2.2 ; ?> M 1 , i = 1.6 M ⊙ , ?> M 2 , i = 0.59 M ⊙ , ?> for [Fe/H] = − 1.5 ?> ) are consistent with the current accepted scenario for the formation of CEMP-s stars.}, number={2}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Placco, Vinicius M. and Beers, Timothy C. and Ivans, Inese I. and Filler, Dan and Imig, Julie A. and Roederer, Ian U. and Abate, Carlo and Hansen, Terese and Cowan, John J. and Frebel, Anna and et al.}, year={2015}, month={Oct}, pages={109} }
 @article{liu_yong_asplund_ramírez_meléndez_gustafsson_howes_roederer_lambert_bensby_2015, title={The detailed chemical composition of the terrestrial planet host Kepler-10}, volume={456}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stv2821}, DOI={10.1093/mnras/stv2821}, abstractNote={Chemical abundance studies of the Sun and solar twins have demonstrated that the solar composition of refractory elements is depleted when compared to volatile elements, which could be due to the formation of terrestrial planets. In order to further examine this scenario, we conducted a line-by-line differential chemical abundance analysis of the terrestrial planet host Kepler-10 and 14 of its stellar twins. Stellar parameters and elemental abundances of Kepler-10 and its stellar twins were obtained with very high precision using a strictly differential analysis of high quality Canada-France-Hawaii Telescope, Hobby-Eberly Telescope and Magellan spectra. When compared to the majority of thick disc twins, Kepler-10 shows a depletion in the refractory elements relative to the volatile elements, which could be due to the formation of terrestrial planets in the Kepler-10 system. The average abundance pattern corresponds to similar to 13 Earth masses, while the two known planets in Kepler-10 system have a combined similar to 20 Earth masses. For two of the eight thick disc twins, however, no depletion patterns are found. Although our results demonstrate that several factors [e.g. planet signature, stellar age, stellar birth location and Galactic chemical evolution (GCE)] could lead to or affect abundance trends with condensation temperature, we find that the trends give further support for the planetary signature hypothesis.}, number={3}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Liu, F. and Yong, D. and Asplund, M. and Ramírez, I. and Meléndez, J. and Gustafsson, B. and Howes, L. M. and Roederer, I. U. and Lambert, D. L. and Bensby, T.}, year={2015}, month={Dec}, pages={2636–2646} }
 @article{roederer_2015, title={Ultraviolet Spectroscopy of Metal-Poor Stars: New Detections of Phosphorus, Germanium, Arsenic, Selenium, Cadmium, Tellurium, Lutetium, Osmium, Iridium, Platinum, Gold, and More!}, volume={225}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2015}, pages={133.01} }
 @article{roederer_preston_thompson_shectman_sneden_burley_kelson_2014, title={A SEARCH FOR STARS OF VERY LOW METAL ABUNDANCE. VI. DETAILED ABUNDANCES OF 313 METAL-POOR STARS}, volume={147}, DOI={10.1088/0004-6256/147/6/136}, abstractNote={We present radial velocities, equivalent widths, model atmosphere parameters, and abundances or upper limits for 53 species of 48 elements derived from high resolution optical spectroscopy of 313 metal-poor stars. A majority of these stars were selected from the metal-poor candidates of the HK Survey of Beers, Preston, and Shectman. We derive detailed abundances for 61% of these stars for the first time. Spectra were obtained during a 10 yr observing campaign using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Telescopes at Las Campanas Observatory, the Robert G. Tull Coudé Spectrograph on the Harlan J. Smith Telescope at McDonald Observatory, and the High Resolution Spectrograph on the Hobby–Eberly Telescope at McDonald Observatory. We perform a standard LTE abundance analysis using MARCS model atmospheres, and we apply line-by-line statistical corrections to minimize systematic abundance differences arising when different sets of lines are available for analysis. We identify several abundance correlations with effective temperature. A comparison with previous abundance analyses reveals significant differences in stellar parameters, which we investigate in detail. Our metallicities are, on average, lower by ≈0.25 dex for red giants and ≈0.04 dex for subgiants. Our sample contains 19 stars with [Fe/H] ⩽−3.5, 84 stars with [Fe/H] ⩽−3.0, and 210 stars with [Fe/H] ⩽−2.5. Detailed abundances are presented here or elsewhere for 91% of the 209 stars with [Fe/H] ⩽−2.5 as estimated from medium resolution spectroscopy by Beers, Preston, and Shectman. We will discuss the interpretation of these abundances in subsequent papers.}, number={6}, journal={The Astronomical Journal}, publisher={IOP Publishing}, author={Roederer, Ian U. and Preston, George W. and Thompson, Ian B. and Shectman, Stephen A. and Sneden, Christopher and Burley, Gregory S. and Kelson, Daniel D.}, year={2014}, month={May}, pages={136} }
 @article{yong_alves brito_da costa_alonso-garcía_karakas_pignatari_roederer_aoki_fishlock_grundahl_et al._2014, title={Chemical abundances in bright giants of the globular cluster M62 (NGC 6266)★}, volume={439}, ISSN={1365-2966 0035-8711}, url={http://dx.doi.org/10.1093/mnras/stu118}, DOI={10.1093/mnras/stu118}, abstractNote={With the exception of Terzan 5, all the Galactic globular clusters that possess significant metallicity spreads, such as ω Cen and M22, are preferentially the more luminous clusters with extended horizontal branches. Here we present radial velocities and chemical abundances for seven bright giants in the globular cluster M62, a previously little-studied cluster. With MV = −9.18, M62 is the ninth most luminous Galactic globular cluster and has an extended horizontal branch. Within our sample, we find (i) no evidence for a dispersion in metallicity, [Fe/H], beyond the measurement uncertainties, (ii) star-to-star abundance variations for C, O, Na and Al with the usual correlations between these elements as seen in other globular clusters, and (iii) a global enrichment for the elements Zr, Ba and La at the level [X/Fe] ≃ +0.4 dex. For elements heavier than La, the abundance ratios are consistent with the scaled-solar r-process distribution. Below La, the abundances are anomalous when compared to the scaled-solar s-process or r-process distributions. For these elements, the abundance signature in M62 is in agreement with predictions of the s-process from fast-rotating massive stars, although the high [Rb/Y] ratio we measure may be a challenge to this scenario.}, number={3}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Yong, David and Alves Brito, Alan and Da Costa, Gary S. and Alonso-García, Javier and Karakas, Amanda I. and Pignatari, Marco and Roederer, Ian U. and Aoki, Wako and Fishlock, Cherie K. and Grundahl, Frank and et al.}, year={2014}, month={Feb}, pages={2638–2650} }
 @article{roederer_jacobson_thanathibodee_frebel_toller_2014, title={DETECTION OF NEUTRAL PHOSPHORUS IN THE NEAR-ULTRAVIOLET SPECTRA OF LATE-TYPE STARS}, volume={797}, DOI={10.1088/0004-637X/797/1/69}, abstractNote={We report the detection of several absorption lines of neutral phosphorus (P, Z = 15) in archival near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We derive phosphorus abundances or interesting upper limits in 14 late-type stars with metallicities spanning −3.8 < [Fe/H] <−0.1. Previously, phosphorus had only been studied in Galactic stars with −1.0 < [Fe/H] <+0.3. Iron lines reveal abundance offsets between the optical and ultraviolet regions, and we discuss and apply a correction factor to account for this offset. In stars with [Fe/H] >−1.0, the [P/Fe] ratio decreases toward the solar value with increasing metallicity, in agreement with previous observational studies. In stars with [Fe/H] <−1.0, 〈[P/Fe]〉 = +0.04 ± 0.10, which overlaps with the [P/Fe] ratios found in several high-redshift damped Lyman-α systems. This behavior hints at a primary origin in massive stars.}, number={1}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Roederer, Ian U. and Jacobson, Heather R. and Thanathibodee, Thanawuth and Frebel, Anna and Toller, Elizabeth}, year={2014}, month={Nov}, pages={69} }
 @article{roederer_kirby_2014, title={Detailed abundance analysis of the brightest star in Segue 2, the least massive galaxy★}, volume={440}, ISSN={1365-2966 0035-8711}, url={http://dx.doi.org/10.1093/mnras/stu491}, DOI={10.1093/mnras/stu491}, abstractNote={We present the first high resolution spectroscopic observations of one red giant star in the ultra-faint dwarf galaxy Segue 2, which has the lowest total mass (including dark matter) estimated for any known galaxy. These observations were made using the MIKE spectrograph on the Magellan II Telescope at Las Campanas Observatory. We perform a standard abundance analysis of this star, SDSS J021933.13+200830.2, and present abundances of 21 species of 18 elements as well as upper limits for 25 additional species. We derive [Fe/H] = -2.9, in excellent agreement with previous estimates from medium resolution spectroscopy. Our main result is that this star bears the chemical signatures commonly found in field stars of similar metallicity. The heavy elements produced by neutron-capture reactions are present, but they are deficient at levels characteristic of stars in other ultra-faint dwarf galaxies and a few luminous dwarf galaxies. The otherwise normal abundance patterns suggest that the gas from which this star formed was enriched by metals from multiple Type II supernovae reflecting a relatively well-sampled IMF. This adds to the growing body of evidence indicating that Segue 2 may have been substantially more massive in the past.}, number={3}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Roederer, Ian U. and Kirby, Evan N.}, year={2014}, month={Apr}, pages={2665–2675} }
 @article{ramírez_bajkova_bobylev_roederer_lambert_endl_cochran_macqueen_wittenmyer_2014, title={ELEMENTAL ABUNDANCES OF SOLAR SIBLING CANDIDATES}, volume={787}, DOI={10.1088/0004-637X/787/2/154}, abstractNote={Dynamical information along with survey data on metallicity and in some cases age have been used recently by some authors to search for candidates of stars that were born in the cluster where the Sun formed. We have acquired high-resolution, high signal-to-noise ratio spectra for 30 of these objects to determine, using detailed elemental abundance analysis, if they could be true solar siblings. Only two of the candidates are found to have solar chemical composition. Updated modeling of the stars' past orbits in a realistic Galactic potential reveals that one of them, HD 162826, satisfies both chemical and dynamical conditions for being a sibling of the Sun. Measurements of rare-element abundances for this star further confirm its solar composition, with the only possible exception of Sm. Analysis of long-term high-precision radial velocity data rules out the presence of hot Jupiters and confirms that this star is not in a binary system. We find that chemical tagging does not necessarily benefit from studying as many elements as possible but instead from identifying and carefully measuring the abundances of those elements that show large star-to-star scatter at a given metallicity. Future searches employing data products from ongoing massive astrometric and spectroscopic surveys can be optimized by acknowledging this fact.}, number={2}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Ramírez, I. and Bajkova, A. T. and Bobylev, V. V. and Roederer, I. U. and Lambert, D. L. and Endl, M. and Cochran, W. D. and MacQueen, P. J. and Wittenmyer, R. A.}, year={2014}, month={May}, pages={154} }
 @article{placco_beers_roederer_cowan_frebel_filler_ivans_lawler_schatz_sneden_et al._2014, title={HUBBLE SPACE TELESCOPE</i>NEAR-ULTRAVIOLET SPECTROSCOPY OF THE BRIGHT CEMP-NO STAR BD+44°493}, volume={790}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/790/1/34}, DOI={10.1088/0004-637x/790/1/34}, abstractNote={We present an elemental-abundance analysis, in the near-ultraviolet (NUV) spectral range, for the extremely metal-poor star BD+44°493 a ninth magnitude subgiant with [Fe/H] =−3.8 and enhanced carbon, based on data acquired with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. This star is the brightest example of a class of objects that, unlike the great majority of carbon-enhanced metal-poor (CEMP) stars, does not exhibit over-abundances of heavy neutron-capture elements (CEMP-no). In this paper, we validate the abundance determinations for a number of species that were previously studied in the optical region, and obtain strong upper limits for beryllium and boron, as well as for neutron-capture elements from zirconium to platinum, many of which are not accessible from ground-based spectra. The boron upper limit we obtain for BD+44°493, log ϵ (B) <−0.70, the first such measurement for a CEMP star, is the lowest yet found for very and extremely metal-poor stars. In addition, we obtain even lower upper limits on the abundances of beryllium, log ϵ (Be) <−2.3, and lead, log ϵ (Pb) <−0.23 ([Pb/Fe] <+1.90), than those reported by previous analyses in the optical range. Taken together with the previously measured low abundance of lithium, the very low upper limits on Be and B suggest that BD+44°493 was formed at a very early time, and that it could well be a bona-fide second-generation star. Finally, the Pb upper limit strengthens the argument for non-s-process production of the heavy-element abundance patterns in CEMP-no stars.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Placco, Vinicius M. and Beers, Timothy C. and Roederer, Ian U. and Cowan, John J. and Frebel, Anna and Filler, Dan and Ivans, Inese I. and Lawler, James E. and Schatz, Hendrik and Sneden, Christopher and et al.}, year={2014}, month={Jun}, pages={34} }
 @article{yong_roederer_grundahl_da costa_karakas_norris_aoki_fishlock_marino_milone_et al._2014, title={Iron and neutron-capture element abundance variations in the globular cluster M2 (NGC 7089)★}, volume={441}, ISSN={1365-2966 0035-8711}, url={http://dx.doi.org/10.1093/mnras/stu806}, DOI={10.1093/mnras/stu806}, abstractNote={We present CN and CH indices and CaII triplet metallicities for 34 giant stars and chemical abundances for 33 elements in 14 giants in the globular cluster M2. Assuming the program stars are cluster members, our analysis reveals (i) an extreme variation in CN and CH line strengths, (ii) a metallicity dispersion with a dominant peak at [Fe/H] = -1.7 and smaller peaks at -1.5 and -1.0, (iii) star-to-star abundance variations and correlations for the light elements O, Na, Al and Si and (iv) a large (and possibly bimodal) distribution in the abundances of all elements produced mainly via the s-process in solar system material. Following Roederer et al. (2011), we define two groups of stars, "r+s" and "r-only", and subtract the average abundances of the latter from the former group to obtain a "s-process residual". This s-process residual is remarkably similar to that found in M22 and in M4 despite the range in metallicity covered by these three systems. With recent studies identifying a double subgiant branch in M2 and a dispersion in Sr and Ba abundances, our spectroscopic analysis confirms that this globular cluster has experienced a complex formation history with similarities to M22, NGC 1851 and omega Centauri.}, number={4}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Yong, David and Roederer, Ian U. and Grundahl, Frank and Da Costa, Gary S. and Karakas, Amanda I. and Norris, John E. and Aoki, Wako and Fishlock, Cherie K. and Marino, A. F. and Milone, A. P. and et al.}, year={2014}, month={May}, pages={3396–3416} }
 @article{roederer_preston_thompson_shectman_sneden_2014, title={NEUTRON-CAPTURE NUCLEOSYNTHESIS IN THE FIRST STARS}, volume={784}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/784/2/158}, DOI={10.1088/0004-637x/784/2/158}, abstractNote={Recent studies suggest that metal-poor stars enhanced in carbon but containing low levels of neutron-capture elements may have been among the first to incorporate the nucleosynthesis products of the first generation of stars. We have observed 16 stars with enhanced carbon or nitrogen using the MIKE Spectrograph on the Magellan Telescopes at Las Campanas Observatory and the Tull Spectrograph on the Smith Telescope at McDonald Observatory. We present radial velocities, stellar parameters, and detailed abundance patterns for these stars. Strontium, yttrium, zirconium, barium, europium, ytterbium, and other heavy elements are detected. In four stars, these heavy elements appear to have originated in some form of r-process nucleosynthesis. In one star, a partial s-process origin is possible. The origin of the heavy elements in the rest of the sample cannot be determined unambiguously. The presence of elements heavier than the iron group offers further evidence that zero-metallicity rapidly rotating massive stars and pair instability supernovae did not contribute substantial amounts of neutron-capture elements to the regions where the stars in our sample formed. If the carbon- or nitrogen-enhanced metal-poor stars with low levels of neutron-capture elements were enriched by products of zero-metallicity supernovae only, then the presence of these heavy elements indicates that at least one form of neutron-capture reaction operated in some of the first stars.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Preston, George W. and Thompson, Ian B. and Shectman, Stephen A. and Sneden, Christopher}, year={2014}, month={Mar}, pages={158} }
 @article{roederer_schatz_lawler_beers_cowan_frebel_ivans_sneden_sobeck_2014, title={NEW DETECTIONS OF ARSENIC, SELENIUM, AND OTHER HEAVY ELEMENTS IN TWO METAL-POOR STARS}, volume={791}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/791/1/32}, DOI={10.1088/0004-637x/791/1/32}, abstractNote={We use the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope to obtain new high-quality spectra covering the 1900 ⩽λ ⩽ 2360 Å wavelength range for two metal-poor stars, HD 108317 and HD 128279. We derive abundances of Cu ii, Zn ii, As i, Se i, Mo ii, and Cd ii, which have not been detected previously in either star. Abundances derived for Ge i, Te i, Os ii, and Pt i confirm those derived from lines at longer wavelengths. We also derive upper limits from the non-detection of W ii, Hg ii, Pb ii, and Bi i. The mean [As/Fe] ratio derived from these two stars and five others in the literature is unchanged over the metallicity range −2.8 < [Fe/H] <−0.6, 〈[As/Fe]〉 = +0.28 ± 0.14 (σ = 0.36 dex). The mean [Se/Fe] ratio derived from these two stars and six others in the literature is also constant, 〈[Se/Fe]〉 = +0.16 ± 0.09 (σ = 0.26 dex). The As and Se abundances are enhanced relative to a simple extrapolation of the iron-peak abundances to higher masses, suggesting that this mass region (75 ⩽A ⩽ 82) may be the point at which a different nucleosynthetic mechanism begins to dominate the quasi-equilibrium α-rich freezeout of the iron peak. 〈[Cu ii/Cu i]〉 = +0.56 ± 0.23 in HD 108317 and HD 128279, and we infer that lines of Cu i may not be formed in local thermodynamic equilibrium in these stars. The [Zn/Fe], [Mo/Fe], [Cd/Fe], and [Os/Fe] ratios are also derived from neutral and ionized species, and each ratio pair agrees within the mutual uncertainties, which range from 0.15 to 0.52 dex.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Schatz, Hendrik and Lawler, James E. and Beers, Timothy C. and Cowan, John J. and Frebel, Anna and Ivans, Inese I. and Sneden, Christopher and Sobeck, Jennifer S.}, year={2014}, month={Jul}, pages={32} }
 @inproceedings{roederer_2014, title={New Observations of Elements with 70 < A < 90}, booktitle={Torino Workshop on Nucleosynthesis in AGB Stars}, author={Roederer, I.U.}, year={2014} }
 @inproceedings{roederer,_2014, title={New Observations of r-process Material in Metal-Poor Stars}, booktitle={Workshop on the Status and Challenges of the r-process, INT}, author={Roederer,, I.U.}, year={2014} }
 @article{roederer_cowan_preston_shectman_sneden_thompson_2014, title={Nine new metal-poor stars on the subgiant and red horizontal branches with high levels of r-process enhancement★}, volume={445}, ISSN={1365-2966 0035-8711}, url={http://dx.doi.org/10.1093/mnras/stu1977}, DOI={10.1093/mnras/stu1977}, abstractNote={We report the discovery of nine metal-poor stars with high levels of r-process enhancement (+0.81<[Eu/Fe]<+1.13), including six subgiants and three stars on the red horizontal branch. We also analyze four previously-known r-process-enhanced metal-poor red giants. From this sample of 13 stars, we draw the following conclusions. (1) High levels of r-process enhancement are found in a broad range of stellar evolutionary states, reaffirming that this phenomenon is not associated with a chemical peculiarity of red giant atmospheres. (2) Only 1 of 10 stars observed at multiple epochs shows radial velocity variations, reaffirming that stars with high levels of r-process enhancement are not preferentially found among binaries. (3) Only 2 of the 13 stars are highly-enhanced in C and N, indicating that there is no connection between high levels of r-process enhancement and high levels of C and N. (4) The dispersions in [Sr/Ba] and [Sr/Eu] are larger than the dispersions in [Ba/Eu] and [Yb/Eu], suggesting that the elements below the second r-process peak do not always scale with those in the rare earth domain, even within the class of highly-r-process-enhanced stars. (5) The light-element (12<Z<30) abundances of highly-r-process-enhanced stars are indistinguishable from those with normal levels of r-process material at the limit of our data, 3.5 per cent (0.015 dex) on average. The nucleosynthetic sites responsible for the large r-process enhancements did not produce any detectable light-element abundance signatures distinct from normal core-collapse supernovae.}, number={3}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Roederer, Ian U. and Cowan, John J. and Preston, George W. and Shectman, Stephen A. and Sneden, Christopher and Thompson, Ian B.}, year={2014}, month={Oct}, pages={2970–2984} }
 @article{jacobson_thanathibodee_frebel_roederer_cescutti_matteucci_2014, title={THE CHEMICAL EVOLUTION OF PHOSPHORUS}, volume={796}, url={https://doi.org/10.1088/2041-8205/796/2/L24}, DOI={10.1088/2041-8205/796/2/L24}, abstractNote={Phosphorus is one of the few remaining light elements for which little is known about its nucleosynthetic origin and chemical evolution, given the lack of optical absorption lines in the spectra of long-lived FGK-type stars. We have identified a P i doublet in the near-ultraviolet (2135/2136 Å) that is measurable in stars of low metallicity. Using archival Hubble Space Telescope–Space Telescope Imaging Spectrograph spectra, we have measured P abundances in 13 stars spanning −3.3 ⩽ [Fe/H] ⩽ -0.2, and obtained an upper limit for a star with [Fe/H] ∼ -3.8. Combined with the only other sample of P abundances in solar-type stars in the literature, which spans a range of −1 ⩽ [Fe/H] ⩽ +0.2, we compare the stellar data to chemical evolution models. Our results support previous indications that massive-star P yields may need to be increased by a factor of a few to match stellar data at all metallicities. Our results also show that hypernovae were important contributors to the P production in the early universe. As P is one of the key building blocks of life, we also discuss the chemical evolution of the important elements to life, C–N–O–P–S, together.}, number={2}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Jacobson, Heather R. and Thanathibodee, Thanawuth and Frebel, Anna and Roederer, Ian U. and Cescutti, Gabriele and Matteucci, Francesca}, year={2014}, month={Nov}, pages={L24} }
 @article{roederer_2013, title={Are There Any Stars Lacking Neutron-Capture Elements?}, volume={221}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2013}, pages={232.02} }
 @article{yong_meléndez_grundahl_roederer_norris_milone_marino_coelho_mcarthur_lind_et al._2013, title={High precision differential abundance measurements in globular clusters: chemical inhomogeneities in NGC 6752★}, volume={434}, ISSN={0035-8711 1365-2966}, url={http://dx.doi.org/10.1093/mnras/stt1276}, DOI={10.1093/mnras/stt1276}, abstractNote={We report on a strictly differential line-by-line analysis of high quality UVES spectra of bright giants in the metal-poor globular cluster NGC 6752. We achieved high precision differential chemical abundance measurements for Fe, Na, Si, Ca, Ti, Cr, Ni, Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu and Dy with uncertainties as low as ~0.01 dex (~2%). We obtained the following main results. (1) The observed abundance dispersions are a factor of ~2 larger than the average measurement uncertainty. (2) There are positive correlations, of high statistical significance, between all elements and Na. (3) For any pair of elements, there are positive correlations of high statistical significance, although the amplitudes of the abundance variations are small. Removing abundance trends with effective temperature and/or using a different pair of reference stars does not alter these results. These abundance variations and correlations may reflect a combination of (a) He abundance variations and (b) inhomogeneous chemical evolution in the pre- or proto-cluster environment. Regarding the former, the current constraints on Delta Y from photometry likely preclude He as being the sole explanation. Regarding the latter, the nucleosynthetic source(s) must have synthesised Na, alpha, Fe-peak and neutron-capture elements and in constant amounts for species heavier than Si; no individual object can achieve such nucleosynthesis. We speculate that other, if not all, globular clusters may exhibit comparable abundance variations and correlations to NGC 6752 if subjected to a similarly precise analysis.}, number={4}, journal={Monthly Notices of the Royal Astronomical Society}, publisher={Oxford University Press (OUP)}, author={Yong, David and Meléndez, Jorge and Grundahl, Frank and Roederer, Ian U. and Norris, John E. and Milone, A. P. and Marino, A. F. and Coelho, P. and McArthur, Barbara E. and Lind, K. and et al.}, year={2013}, month={Aug}, pages={3542–3565} }
 @article{roederer_2013, title={Neutron-Capture Nucleosynthesis in the First Stars}, volume={222}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2013}, pages={403.05} }
 @inproceedings{roederer_2013, title={New Stellar Observations of Exotic Heavy Elements}, booktitle={Nuclear Astrophysics Workshop at the NSCL}, author={Roederer, I.U.}, year={2013} }
 @inproceedings{roederer_2013, title={Synthesis of Heavy Elements in the Early Universe}, booktitle={First Korean-American Kavli Frontiers of Science Symposium}, author={Roederer, I.U.}, year={2013} }
 @inproceedings{roederer_2013, title={The s-process in globular cluster M22: hints for higher-mass polluters}, url={http://dx.doi.org/10.22323/1.146.0021}, DOI={10.22323/1.146.0021}, abstractNote={Every cool globular cluster star that has been studied in sufficient detail reveals the presence of elements heavier than the iron-group. Surely that must be an important clue to understanding how and where these neutron-capture elements are synthesized. The more complex globular clusters like M22 provide a new kind of opportunity to study nucleosynthesis patterns produced by the slow neutron-capture reaction, or s-process. This provides a new constraint on the duration of star formation in M22. This proceeding is a transcript, edited only to capture the essence of images shown on the slides, of the talk I presented on the subject of s-process material in M22.}, booktitle={Proceedings of XII International Symposium on Nuclei in the Cosmos — PoS(NIC XII)}, publisher={Sissa Medialab}, author={Roederer, Ian}, year={2013}, month={Apr} }
 @article{roederer_2012, title={ARE THERE ANY STARS LACKING NEUTRON-CAPTURE ELEMENTS? EVIDENCE FROM STRONTIUM AND BARIUM}, volume={145}, ISSN={0004-6256 1538-3881}, url={http://dx.doi.org/10.1088/0004-6256/145/1/26}, DOI={10.1088/0004-6256/145/1/26}, abstractNote={The cosmic dispersion in the abundances of the heavy elements strontium and barium in halo stars is well known. Strontium and barium are detected in most cool, metal-poor giants, but are these elements always detectable? To identify stars that could be considered probable candidates for lacking these elements, I examine the stellar abundance data available in the literature for 1148 field stars and 226 stars in dwarf galaxies, 776 of which have metallicities lower than [Fe/H] <−2.0. Strontium or barium have been detected in all field, globular cluster, and dwarf galaxy environments studied. All upper limits are consistent with the lowest detected ratios of [Sr/H] and [Ba/H]. The frequent appearance of these elements raises the intriguing prospect that at least one kind of neutron-capture reaction operates as often as the nucleosynthesis mechanisms that produce lighter elements, such as magnesium, calcium, or iron, although the yields of heavy elements may be more variable.}, number={1}, journal={The Astronomical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U.}, year={2012}, month={Dec}, pages={26} }
 @inproceedings{roederer_2012, title={Abundances in r-process enriched metal-poor stars from new UV spectra Roederer, I.U}, booktitle={9th Russbach Workshop on Nuclear Astrophysics}, author={Roederer, I.U.}, year={2012} }
 @article{roederer_lawler_2012, title={DETECTION OF ELEMENTS AT ALL THREE<i>r}, volume={750}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/750/1/76}, DOI={10.1088/0004-637x/750/1/76}, abstractNote={We report the first detection of elements at all three r-process peaks in the metal-poor halo star HD 160617. These elements include arsenic and selenium, which have not been detected previously in halo stars, and the elements tellurium, osmium, iridium, and platinum, which have been detected previously. Absorption lines of these elements are found in archive observations made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. We present up-to-date absolute atomic transition probabilities and complete line component patterns for these elements. Additional archival spectra of this star from several ground-based instruments allow us to derive abundances or upper limits of 45 elements in HD 160617, including 27 elements produced by neutron-capture reactions. The average abundances of the elements at the three r-process peaks are similar to the predicted solar system r-process residuals when scaled to the abundances in the rare earth element domain. This result for arsenic and selenium may be surprising in light of predictions that the production of the lightest r-process elements generally should be decoupled from the heavier r-process elements.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Lawler, James E.}, year={2012}, month={Apr}, pages={76} }
 @article{roederer_lawler_cowan_beers_frebel_ivans_schatz_sobeck_sneden_2012, title={DETECTION OF THE SECOND
                    <i>r
                    -PROCESS PEAK ELEMENT TELLURIUM IN METAL-POOR STARS,}, volume={747}, ISSN={2041-8205 2041-8213}, url={http://dx.doi.org/10.1088/2041-8205/747/1/l8}, DOI={10.1088/2041-8205/747/1/l8}, abstractNote={Using near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, we detect neutral tellurium in three metal-poor stars enriched by products of r-process nucleosynthesis, BD +17 3248, HD 108317, and HD 128279. Tellurium (Te, Z = 52) is found at the second r-process peak (A ≈ 130) associated with the N = 82 neutron shell closure, and it has not been detected previously in Galactic halo stars. The derived tellurium abundances match the scaled solar system r-process distribution within the uncertainties, confirming the predicted second peak r-process residuals. These results suggest that tellurium is predominantly produced in the main component of the r-process, along with the rare earth elements.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Lawler, James E. and Cowan, John J. and Beers, Timothy C. and Frebel, Anna and Ivans, Inese I. and Schatz, Hendrik and Sobeck, Jennifer S. and Sneden, Christopher}, year={2012}, month={Feb}, pages={L8} }
 @article{roederer_2012, title={GERMANIUM, ARSENIC, AND SELENIUM ABUNDANCES IN METAL-POOR STARS*}, volume={756}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/756/1/36}, DOI={10.1088/0004-637x/756/1/36}, abstractNote={ABSTRACT
               The elements germanium (Ge, Z = 32), arsenic (As, Z = 33), and selenium (Se, Z = 34) span the transition from charged-particle or explosive synthesis of the iron-group elements to neutron-capture synthesis of heavier elements. Among these three elements, only the chemical evolution of germanium has been studied previously. Here we use archive observations made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope
and observations from several ground-based facilities to study the chemical enrichment histories of seven stars with metallicities −2.6 ⩽ [Fe/H] ⩽ −0.4. We perform a standard abundance analysis of germanium, arsenic, selenium, and several other elements produced by neutron-capture reactions. When combined with previous derivations of germanium abundances in metal-poor stars, our sample reveals an increase in the [Ge/Fe] ratios at higher metallicities. This could mark the onset of the weak s-process contribution to germanium. In contrast, the [As/Fe] and [Se/Fe] ratios remain roughly constant. These data do not directly indicate the origin of germanium, arsenic, and selenium at low metallicity, but they suggest that the weak and main components of the s-process are not likely sources.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U.}, year={2012}, month={Aug}, pages={36} }
 @misc{mateo_bailey_crane_shectman_thompson_roederer_bigelow_gunnels_2012, title={M2FS: the Michigan/Magellan Fiber System}, ISSN={0277-786X}, url={http://dx.doi.org/10.1117/12.926448}, DOI={10.1117/12.926448}, abstractNote={We describe the Michigan/Magellan Fiber System (M2FS) under construction for use on the Magellan/Clay telescope. M2FS consists of four primary components including: (1) A fiber-fed double spectrograph (MSPec) in which each spectrograph is fed by 128 fibers (for a total multiplexing factor of 256) and each is optimized in to operate from 370- 950 nm; (2) A fiber mounting system (MFib) that supports the fibers and fiber plug plates at the telescope f/11 Nasmyth focal surface and organizes the fibers into ‘shoes’ that are used to place the fibers at the image surface of the MSpec spectrographs;, (3) A new wide-field corrector (WFC) that produces high-quality images over a 30 arcmin diameter field; (4) A unit (MCal) mounted near the telescope secondary that provides wavelength and continuum calibration and that supports a key component in a novel automated fiber identification system. We describe the opto-mechanical properties of M2FS, its modes of operation, and its anticipated performance, as well as potential upgrades including the development of a robotic fiber positioner and an atmospheric dispersion corrector. We describe how the M2FS design could serve as the basis of a powerful wide-field, massively multiplexed spectroscopic survey facility.}, journal={SPIE Proceedings}, publisher={SPIE}, author={Mateo, Mario and Bailey, John I. and Crane, Jeffrey and Shectman, Stephen and Thompson, Ian and Roederer, Ian and Bigelow, Bruce and Gunnels, Steve}, editor={McLean, Ian S. and Ramsay, Suzanne K. and Takami, HidekiEditors}, year={2012}, month={Sep} }
 @article{roederer_lawler_sobeck_beers_cowan_frebel_ivans_schatz_sneden_thompson_2012, title={NEW
                    <i>HUBBLE SPACE TELESCOPE
                    OBSERVATIONS OF HEAVY ELEMENTS IN FOUR METAL-POOR STARS}, volume={203}, ISSN={0067-0049 1538-4365}, url={http://dx.doi.org/10.1088/0067-0049/203/2/27}, DOI={10.1088/0067-0049/203/2/27}, abstractNote={Elements heavier than the iron group are found in nearly all halo stars. A substantial number of these elements, key to understanding neutron-capture nucleosynthesis mechanisms, can only be detected in the near-ultraviolet. We report the results of an observing campaign using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope to study the detailed heavy-element abundance patterns in four metal-poor stars. We derive abundances or upper limits from 27 absorption lines of 15 elements produced by neutron-capture reactions, including seven elements (germanium, cadmium, tellurium, lutetium, osmium, platinum, and gold) that can only be detected in the near-ultraviolet. We also examine 202 heavy-element absorption lines in ground-based optical spectra obtained with the Magellan Inamori Kyocera Echelle Spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory and the High Resolution Echelle Spectrometer on the Keck I Telescope on Mauna Kea. We have detected up to 34 elements heavier than zinc. The bulk of the heavy elements in these four stars are produced by r-process nucleosynthesis. These observations affirm earlier results suggesting that the tellurium found in metal-poor halo stars with moderate amounts of r-process material scales with the rare earth and third r-process peak elements. Cadmium often follows the abundances of the neighboring elements palladium and silver. We identify several sources of systematic uncertainty that must be considered when comparing these abundances with theoretical predictions. We also present new isotope shift and hyperfine structure component patterns for Lu ii and Pb i lines of astrophysical interest.}, number={2}, journal={The Astrophysical Journal Supplement Series}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Lawler, James E. and Sobeck, Jennifer S. and Beers, Timothy C. and Cowan, John J. and Frebel, Anna and Ivans, Inese I. and Schatz, Hendrik and Sneden, Christopher and Thompson, Ian B.}, year={2012}, month={Nov}, pages={27} }
 @inproceedings{roederer_2012, title={New Observational Perspectives on r-process Nucleosynthesis}, booktitle={American Physical Society, 2012 Fall Meeting of the APS Division of Nuclear Physics}, author={Roederer, I.U.}, year={2012} }
 @article{pietrzyński_thompson_gieren_graczyk_stępień_bono_moroni_pilecki_udalski_soszyński_et al._2012, title={RR-Lyrae-type pulsations from a 0.26-solar-mass star in a binary system}, volume={484}, ISSN={0028-0836 1476-4687}, url={http://dx.doi.org/10.1038/nature10966}, DOI={10.1038/nature10966}, abstractNote={The pulsating star OGLE-BLG-RRLYR-02792 is known to be a member of an eclipsing binary system, and its mass is now determined to be only 0.26 times that of the Sun, meaning that it cannot be a classical RR Lyrae pulsator. Astronomers use pulsating variable stars of the RR Lyrae type as indicators of the ages of galaxies, and as tools to measure distances to nearby galaxies. So the news that one of these stars had apparently been found as part of an eclipsing binary system was welcome: it meant that the mass of one of these pulsators, previously available only from models, could be unambiguously determined. But the story is not that simple. Pietrzyński et al. have now determined that the star in question, known as RRLYR-02792, has a mass 0.26 times that of the Sun. This means that it is not a classical RR Lyrae star. Instead, it seems to be a pulsator with observational properties temporarily similar to those of classical RR Lyrae stars, but with different stellar parameters and a different evolutionary history as part of a close binary. The authors estimate that 0.2% of samples of RR Lyrae variables may by contaminated by systems similar to this one, so distances previously measured using RR Lyrae stars should not be significantly affected by the presence of these binaries. RR Lyrae pulsating stars have been extensively used as tracers of old stellar populations for the purpose of determining the ages of galaxies, and as tools to measure distances to nearby galaxies1,2,3. There was accordingly considerable interest when the RR Lyrae star OGLE-BLG-RRLYR-02792 (referred to here as RRLYR-02792) was found to be a member of an eclipsing binary system4, because the mass of the pulsator (hitherto constrained only by models) could be unambiguously determined. Here we report that RRLYR-02792 has a mass of 0.26 solar masses ( ) and therefore cannot be a classical RR Lyrae star. Using models, we find that its properties are best explained by the evolution of a close binary system that started with and stars orbiting each other with an initial period of 2.9 days. Mass exchange over 5.4 billion years produced the observed system, which is now in a very short-lived phase where the physical properties of the pulsator happen to place it in the same instability strip of the Hertzsprung–Russell diagram as that occupied by RR Lyrae stars. We estimate that only 0.2 per cent of RR Lyrae stars may be contaminated by systems similar to this one, which implies that distances measured with RR Lyrae stars should not be significantly affected by these binary interlopers.}, number={7392}, journal={Nature}, publisher={Springer Science and Business Media LLC}, author={Pietrzyński, G. and Thompson, I. B. and Gieren, W. and Graczyk, D. and Stępień, K. and Bono, G. and Moroni, P. G. Prada and Pilecki, B. and Udalski, A. and Soszyński, I. and et al.}, year={2012}, month={Apr}, pages={75–77} }
 @article{marino_milone_sneden_bergemann_kraft_wallerstein_cassisi_aparicio_asplund_bedin_et al._2012, title={The double sub-giant branch of NGC 6656 (M 22): a chemical characterization}, volume={541}, ISSN={0004-6361 1432-0746}, url={http://dx.doi.org/10.1051/0004-6361/201118381}, DOI={10.1051/0004-6361/201118381}, abstractNote={We present an abundance analysis of 101 subgiant branch (SGB) stars in the globular cluster M 22. Using low-resolution FLAMES/GIRAFFE spectra we have determined abundances of the neutron-capture strontium and barium and the light element carbon. With these data we explore relationships between the observed SGB photometric split in this cluster and two stellar groups characterized by different contents of iron, slow neutron-capture process (s-process) elements, and the α element calcium, which we previously discovered in M 22’s red-giant stars. We show that the SGB stars correlate in chemical composition and the color– magnitude diagram position. The stars with higher metallicity and relative s-process abundances define a fainter SGB, while stars with lower metallicity and s-process content reside on a relatively brighter SGB. This result has implications for the relative ages of the two stellar groups of M 22. In particular, it is inconsistent with a broad spread in ages of the two SGBs. By accounting for the chemical content of the two stellar groups, isochrone fitting of the double SGB suggests that their ages are not different by more than ∼300 Myr.}, journal={Astronomy & Astrophysics}, publisher={EDP Sciences}, author={Marino, A. F. and Milone, A. P. and Sneden, C. and Bergemann, M. and Kraft, R. P. and Wallerstein, G. and Cassisi, S. and Aparicio, A. and Asplund, M. and Bedin, R. L. and et al.}, year={2012}, month={Apr}, pages={A15} }
 @inproceedings{roederer_2012, title={r-process Dispersion in Metal-Poor Globular Clusters}, booktitle={The Chemical Evolution of the Milky Way}, author={Roederer, I.U.}, year={2012} }
 @inproceedings{roederer_2011, title={A Range of Neutron-Capture Abundance Ratios Produced by the r-Process}, url={http://dx.doi.org/10.22323/1.100.0281}, DOI={10.22323/1.100.0281}, abstractNote={How varied are the abundance patterns of the heaviest elements observed in metal-poor stars, and how can this diversity inform our understanding of the assembly of matter on the smallest (i.e., nuclear) and largest (i.e., galactic) scales?With regard to the r-process, recent work has shown that (1) there is a dispersion of abundance ratios among the rare earth elements produced in the rprocess, (2) there is also a dispersion in Y/Eu that is not random but (anti-)correlates with [Eu/Fe], and (3) nearly all metal-poor stars contain detectable traces of Sr and Ba that is not a result of s-process nucleosynthesis.If these facts suggest that charged-particle reactions and r-process nucleosynthesis are common phenomena in core-collapse supernovae, then charged-particle and r-process nucleosynthesis products are inextricably linked to supernova physics.Insofar as this physics can be characterized, it may then be a useful tracer of standard chemical evolution rather than its infrequent exception.}, booktitle={Proceedings of 11th Symposium on Nuclei in the Cosmos — PoS(NIC XI)}, publisher={Sissa Medialab}, author={Roederer, Ian}, year={2011}, month={Jul} }
 @article{roederer_preston_shectman_thompson_sneden_2011, title={Abundance Results from the Las Campanas Observatory and McDonald Observatory High-Resolution Metal-Poor Star Survey}, volume={43}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U. and Preston, G. and Shectman, S. and Thompson, I. and Sneden, C.}, year={2011}, pages={154.03} }
 @article{roederer_marino_sneden_2011, title={CHARACTERIZING THE HEAVY ELEMENTS IN GLOBULAR CLUSTER M22 AND AN EMPIRICAL<i>s</i>-PROCESS ABUNDANCE DISTRIBUTION DERIVED FROM THE TWO STELLAR GROUPS}, volume={742}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/742/1/37}, DOI={10.1088/0004-637x/742/1/37}, abstractNote={We present an empirical s-process abundance distribution derived with explicit knowledge of the r-process component in the low-metallicity globular cluster M22. We have obtained high-resolution, high signal-to-noise spectra for six red giants in M22 using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan-Clay Telescope at Las Campanas Observatory. In each star we derive abundances for 44 species of 40 elements, including 24 elements heavier than zinc (Z = 30) produced by neutron-capture reactions. Previous studies determined that three of these stars (the “r+s group”) have an enhancement of s-process material relative to the other three stars (the “r-only group”). We confirm that the r+s group is moderately enriched in Pb relative to the r-only group. Both groups of stars were born with the same amount of r-process material, but s-process material was also present in the gas from which the r+s group formed. The s-process abundances are inconsistent with predictions for asymptotic giant branch (AGB) stars with M ⩽ 3 M☉ and suggest an origin in more massive AGB stars capable of activating the 22Ne(α,n)25Mg reaction. We calculate the s-process “residual” by subtracting the r-process pattern in the r-only group from the abundances in the r+s group. In contrast to previous r- and s-process decompositions, this approach makes no assumptions about the r- and s-process distributions in the solar system and provides a unique opportunity to explore s-process yields in a metal-poor environment.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, I. U. and Marino, A. F. and Sneden, C.}, year={2011}, month={Nov}, pages={37} }
 @article{ramírez_meléndez_cornejo_roederer_fish_2011, title={ELEMENTAL ABUNDANCE DIFFERENCES IN THE 16 CYGNI BINARY SYSTEM: A SIGNATURE OF GAS GIANT PLANET FORMATION?}, volume={740}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/740/2/76}, DOI={10.1088/0004-637x/740/2/76}, abstractNote={The atmospheric parameters of the components of the 16 Cygni binary system, in which the secondary has a gas giant planet detected, are measured accurately using high-quality observational data. Abundances relative to solar are obtained for 25 elements with a mean error of σ([X/H]) = 0.023 dex. The fact that 16 Cyg A has about four times more lithium than 16 Cyg B is normal considering the slightly different masses of the stars. The abundance patterns of 16 Cyg A and B, relative to iron, are typical of that observed in most of the so-called solar twin stars, with the exception of the heavy elements (Z > 30), which can, however, be explained by Galactic chemical evolution. Differential (A–B) abundances are measured with even higher precision (σ(Δ[X/H]) = 0.018 dex, on average). We find that 16 Cyg A is more metal-rich than 16 Cyg B by Δ[M/H] = +0.041 ± 0.007 dex. On an element-to-element basis, no correlation between the A–B abundance differences and dust condensation temperature (TC) is detected. Based on these results, we conclude that if the process of planet formation around 16 Cyg B is responsible for the observed abundance pattern, the formation of gas giants produces a constant downward shift in the photospheric abundance of metals, without a TC correlation. The latter would be produced by the formation of terrestrial planets instead, as suggested by other recent works on precise elemental abundances. Nevertheless, a scenario consistent with these observations requires the convective envelopes of ≃ 1 M☉ stars to reach their present-day sizes about three times quicker than predicted by standard stellar evolution models.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Ramírez, I. and Meléndez, J. and Cornejo, D. and Roederer, I. U. and Fish, J. R.}, year={2011}, month={Sep}, pages={76} }
 @article{roederer_sneden_2011, title={HEAVY-ELEMENT DISPERSION IN THE METAL-POOR GLOBULAR CLUSTER M92}, volume={142}, ISSN={0004-6256 1538-3881}, url={http://dx.doi.org/10.1088/0004-6256/142/1/22}, DOI={10.1088/0004-6256/142/1/22}, abstractNote={Dispersion among the light elements is common in globular clusters (GCs), while dispersion among heavier elements is less common. We present detection of r-process dispersion relative to Fe in 19 red giants of the metal-poor GC M92. Using spectra obtained with the Hydra multi-object spectrograph on the WIYN Telescope at Kitt Peak National Observatory, we derive differential abundances for 21 species of 19 elements. The Fe-group elements, plus Y and Zr, are homogeneous at a level of 0.07–0.16 dex. The heavy-elements La, Eu, and Ho exhibit clear star-to-star dispersion spanning 0.5–0.8 dex. The abundances of these elements are correlated with one another, and we demonstrate that they were produced by r-process nucleosynthesis. This r-process dispersion is not correlated with the dispersion in C, N, or Na in M92, indicating that r-process inhomogeneities were present in the gas throughout star formation. The r-process dispersion is similar to that previously observed in the metal-poor GC M15, but its origin in M15 or M92 is unknown at present.}, number={1}, journal={The Astronomical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Sneden, Christopher}, year={2011}, month={Jun}, pages={22} }
 @article{roederer_2011, title={PRIMORDIAL
                    <i>r
                    -PROCESS DISPERSION IN METAL-POOR GLOBULAR CLUSTERS}, volume={732}, ISSN={2041-8205 2041-8213}, url={http://dx.doi.org/10.1088/2041-8205/732/1/l17}, DOI={10.1088/2041-8205/732/1/l17}, abstractNote={Heavy elements, those produced by neutron-capture reactions, have traditionally shown no star-to-star dispersion in all but a handful of metal-poor globular clusters (GCs). Recent detections of low [Pb/Eu] ratios or upper limits in several metal-poor GCs indicate that the heavy elements in these GCs were produced exclusively by an r-process. Re-examining GC heavy element abundances from the literature, we find unmistakable correlations between the [La/Fe] and [Eu/Fe] ratios in four metal-poor GCs (M5, M15, M92, and NGC 3201), only two of which were known previously. This indicates that the total r-process abundances vary from star to star (by factors of 2–6) relative to Fe within each GC. We also identify potential dispersion in two other GCs (M3 and M13). Several GCs (M12, M80, and NGC 6752) show no evidence of r-process dispersion. The r-process dispersion is not correlated with the well-known light element dispersion, indicating that it was present in the gas throughout the duration of star formation. The observations available at present suggest that star-to-star r-process dispersion within metal-poor GCs may be a common but not ubiquitous phenomenon that is neither predicted by nor accounted for in current models of GC formation and evolution.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U.}, year={2011}, month={Apr}, pages={L17} }
 @article{hollek_frebel_roederer_sneden_shetrone_beers_kang_thom_2011, title={THE CHEMICAL ABUNDANCES OF STARS IN THE HALO (CASH) PROJECT. II. A SAMPLE OF 14 EXTREMELY METAL-POOR STARS,}, volume={742}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/742/1/54}, DOI={10.1088/0004-637x/742/1/54}, abstractNote={We present a comprehensive abundance analysis of 20 elements for 16 new low-metallicity stars from the Chemical Abundances of Stars in the Halo (CASH) project. The abundances have been derived from both Hobby-Eberly Telescope High Resolution Spectrograph snapshot spectra (R ∼15, 000) and corresponding high-resolution (R ∼35, 000) Magellan Inamori Kyocera Echelle spectra. The stars span a metallicity range from [Fe/H] from −2.9 to −3.9, including four new stars with [Fe/H] < −3.7. We find four stars to be carbon-enhanced metal-poor (CEMP) stars, confirming the trend of increasing [C/Fe] abundance ratios with decreasing metallicity. Two of these objects can be classified as CEMP-no stars, adding to the growing number of these objects at [Fe/H]< − 3. We also find four neutron-capture-enhanced stars in the sample, one of which has [Eu/Fe] of 0.8 with clear r-process signatures. These pilot sample stars are the most metal-poor ([Fe/H] ≲ −3.0) of the brightest stars included in CASH and are used to calibrate a newly developed, automated stellar parameter and abundance determination pipeline. This code will be used for the entire ∼500 star CASH snapshot sample. We find that the pipeline results are statistically identical for snapshot spectra when compared to a traditional, manual analysis from a high-resolution spectrum.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Hollek, Julie K. and Frebel, Anna and Roederer, Ian U. and Sneden, Christopher and Shetrone, Matthew and Beers, Timothy C. and Kang, Sung-ju and Thom, Christopher}, year={2011}, month={Nov}, pages={54} }
 @article{boyd_famiano_meyer_motizuki_kajino_roederer_2011, title={THE r-PROCESS IN METAL-POOR STARS AND BLACK HOLE FORMATION}, volume={744}, ISSN={2041-8205 2041-8213}, url={http://dx.doi.org/10.1088/2041-8205/744/1/l14}, DOI={10.1088/2041-8205/744/1/l14}, abstractNote={Nucleosynthesis of heavy nuclei in metal-poor stars is generally thought to occur via the r-process because the r-process is a primary process that would have operated early in the Galaxy's history. This idea is strongly supported by the fact that the abundance pattern in many metal-poor stars matches well the inferred solar r-process abundance pattern in the mass range between the second and third r-process abundance peaks. Nevertheless, a significant number of metal-poor stars do not share this standard r-process template. In this Letter, we suggest that the nuclides observed in many of these stars are produced by the r-process, but that it is prevented from running to completion in more massive stars by collapse to black holes before the r-process is completed, creating a “truncated r-process,” or “tr-process.” We find that the observed fraction of tr-process stars is qualitatively what one would expect from the initial mass function and that an apparent sharp truncation observed at around mass 160 could result from a combination of collapses to black holes and the difficulty of observing the higher mass rare-earth elements. We test the tr-process hypothesis with r-process calculations that are terminated before all r-process trajectories have been ejected. We find qualitative agreement between observation and theory when black hole collapse and observational realities are taken into account.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Boyd, R. N. and Famiano, M. A. and Meyer, B. S. and Motizuki, Y. and Kajino, T. and Roederer, I. U.}, year={2011}, month={Dec}, pages={L14} }
 @article{roederer_sneden_thompson_preston_shectman_2010, title={CHARACTERIZING THE CHEMISTRY OF THE MILKY WAY STELLAR HALO: DETAILED CHEMICAL ANALYSIS OF A METAL-POOR STELLAR STREAM,}, volume={711}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/711/2/573}, DOI={10.1088/0004-637x/711/2/573}, abstractNote={We present the results of a detailed abundance analysis of one of the confirmed building blocks of the Milky Way stellar halo, a kinematically coherent metal-poor stellar stream. We have obtained high-resolution and high signal-to-noise spectra of 12 probable stream members using the Magellan Inamori Kyocera Echelle spectrograph on the Magellan–Clay Telescope at Las Campanas Observatory and the 2dCoude spectrograph on the Smith Telescope at McDonald Observatory. We have derived abundances or upper limits for 51 species of 46 elements in each of these stars. The stream members show a range of metallicity (−3.4 < [Fe/H] <−1.5) but are otherwise chemically homogeneous, with the same star-to-star dispersion in [X/Fe] as the rest of the halo. This implies that, in principle, a significant fraction of the Milky Way stellar halo could have formed from accreted systems like the stream. The stream stars show minimal evolution in the α or Fe-group elements over the range of metallicity. This stream is enriched with material produced by the main and weak components of the rapid neutron-capture process and shows no evidence for enrichment by the slow neutron-capture process.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Sneden, Christopher and Thompson, Ian B. and Preston, George W. and Shectman, Stephen A.}, year={2010}, month={Feb}, pages={573–596} }
 @article{roederer_2010, title={Characterizing the Chemistry of the Milky Way Stellar Halo}, volume={42}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2010}, pages={320} }
 @inproceedings{roederer_sneden_thompson_preston_shectman_2010, place={San Francisco, CA}, series={ASP Conerence Series}, title={Detailed Abundances in a Metal-Poor Stellar Stream}, volume={432}, booktitle={New Horizons in Astronomy: Frank N. Bash Symposium 2009, proceedings of a conference held Oct. 18-20, 2009 in Austin, Texas, USA}, publisher={Astronomical Society of the Pacific}, author={Roederer, I.U. and Sneden, C. and Thompson, I.B. and Preston, G.W. and Shectman, S.A.}, editor={Stanford, L.M. and Green, J.D. and Hai, L. and Mao, Y.Editors}, year={2010}, pages={239}, collection={ASP Conerence Series} }
 @article{roederer_sneden_lawler_cowan_2010, title={NEW ABUNDANCE DETERMINATIONS OF CADMIUM, LUTETIUM, AND OSMIUM IN THE
                    <i>r
                    -PROCESS ENRICHED STAR BD +17 3248
                    <sup>,}, volume={714}, ISSN={2041-8205 2041-8213}, url={http://dx.doi.org/10.1088/2041-8205/714/1/l123}, DOI={10.1088/2041-8205/714/1/l123}, abstractNote={We report the detection of Cd i (Z = 48), Lu ii (Z = 71), and Os ii (Z = 76) in the metal-poor star BD +17 3248. These abundances are derived from an ultraviolet spectrum obtained with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. This is the first detection of these neutron-capture species in a metal-poor star enriched by the r process. We supplement these measurements with new abundances of Mo i, Ru i, and Rh i derived from an optical spectrum obtained with the High Resolution Echelle Spectrograph on Keck. Combined with previous abundance derivations, 32 neutron-capture elements have been detected in BD +17 3248, the most complete neutron-capture abundance pattern in any metal-poor star to date. The light neutron-capture elements (38 ⩽ Z ⩽ 48) show a more pronounced even–odd effect than expected from current solar system r-process abundance predictions. The age for BD +17 3248 derived from the Th ii/Os ii chronometer is in better agreement with the age derived from other chronometers than the age derived from Th ii/Os i. New Hf ii abundance derivations from transitions in the ultraviolet are lower than those derived from transitions in the optical, and the lower Hf abundance is in better agreement with the scaled solar system r-process distribution.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Sneden, Christopher and Lawler, James E. and Cowan, John J.}, year={2010}, month={Apr}, pages={L123–L127} }
 @article{roederer_cowan_karakas_kratz_lugaro_simmerer_farouqi_sneden_2010, title={THE UBIQUITY OF THE RAPID NEUTRON-CAPTURE PROCESS*}, volume={724}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/724/2/975}, DOI={10.1088/0004-637x/724/2/975}, abstractNote={ABSTRACT
               To better characterize the abundance patterns produced by the r-process, we have derived new abundances or upper limits for the heavy elements zinc (Zn, Z= 30), yttrium (Y, Z= 39), lanthanum (La, Z= 57), europium (Eu, Z= 63), and lead (Pb, Z= 82). Our sample of 161 metal-poor stars includes new measurements from 88 high-resolution and high signal-to-noise spectra obtained with the Tull Spectrograph on the 2.7 m Smith Telescope at the McDonald Observatory, and other abundances are adopted from the literature. We use models of the s-process in asymptotic giant branch stars to characterize the high Pb/Eu ratios produced in the s-process at low metallicity, and our new observations then allow us to identify a sample of stars with no detectable s-process material. In these stars, we find no significant increase in the Pb/Eu ratios with increasing metallicity. This suggests that s-process material was not widely dispersed until the overall Galactic metallicity grew considerably, perhaps even as high as [Fe/H] =−1.4, in contrast with earlier studies that suggested a much lower mean metallicity. We identify a dispersion of at least 0.5 dex in [La/Eu] in metal-poor stars with [Eu/Fe] <+0.6 attributable to the r-process, suggesting that there is no unique “pure” r-process elemental ratio among pairs of rare earth elements. We confirm earlier detections of an anti-correlation between Y/Eu and Eu/Fe bookended by stars strongly enriched in the r-process (e.g., CS 22892–052) and those with deficiencies of the heavy elements (e.g., HD 122563). We can reproduce the range of Y/Eu ratios using simulations of high-entropy neutrino winds of core-collapse supernovae that include charged-particle and neutron-capture components of r-process nucleosynthesis. The heavy element abundance patterns in most metal-poor stars do not resemble that of CS 22892–052, but the presence of heavy elements such as Ba in nearly all metal-poor stars without s-process enrichment suggests that the r-process is a common phenomenon.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Cowan, John J. and Karakas, Amanda I. and Kratz, Karl-Ludwig and Lugaro, Maria and Simmerer, Jennifer and Farouqi, Khalil and Sneden, Christopher}, year={2010}, month={Nov}, pages={975–993} }
 @article{roederer_2009, title={Chemical Inhomogeneities in the Milky Way Stellar Halo(s)}, volume={41}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U.}, year={2009}, pages={206} }
 @article{roederer_sneden_thompson_preston_shectman_2009, title={Detailed Chemical Abundances in a Metal-Poor Stellar Stream}, volume={5}, ISSN={1743-9213 1743-9221}, url={http://dx.doi.org/10.1017/s1743921310000967}, DOI={10.1017/s1743921310000967}, abstractNote={AbstractWe have observed 9 bright metal-poor stars whose kinematics suggest they are members of a stellar stream in the vicinity of the Solar neighborhood. These 9 stars exhibit no star-to-star dispersion in their [X/Fe] ratios for the α and Fe-peak elements, and the neutron-capture elements suggest mild enrichment by the main r-process. The abundance patterns seen in this stream are very similar to those found in the metal-poor globular cluster M15, and the kinematics of M15 are similar to those of the stream, suggesting that these two groups of stars may have shared a common origin.}, number={S265}, journal={Proceedings of the International Astronomical Union}, publisher={Cambridge University Press (CUP)}, author={Roederer, Ian U. and Sneden, Christopher and Thompson, Ian B. and Preston, George W. and Shectman, Stephen A.}, year={2009}, month={Aug}, pages={368–369} }
 @inproceedings{roederer_kratz_frebel_christlieb_pfeiffer_cowan_sneden_2009, place={Cambridge, UK}, series={IAU symposium proceedings series}, title={Nucleosynthesis of Lead and Thorium in the Early Galaxy}, booktitle={The ages of stars : proceedings of the 258th Symposium of the International Astronomical Union held in Baltimore, Maryland, USA, October 13-17, 2008}, publisher={Cambridge University Press}, author={Roederer, I.U. and Kratz, K.-L. and Frebel, A. and Christlieb, N. and Pfeiffer, B. and Cowan, J.J. and Sneden, C.}, editor={Mamajek, E.E. and Soderblom, D.R. and Wyse, R.F.G.Editors}, year={2009}, pages={455}, collection={IAU symposium proceedings series} }
 @article{roederer_kratz_frebel_christlieb_pfeiffer_cowan_sneden_2009, title={THE END OF NUCLEOSYNTHESIS: PRODUCTION OF LEAD AND THORIUM IN THE EARLY GALAXY}, volume={698}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637x/698/2/1963}, DOI={10.1088/0004-637x/698/2/1963}, abstractNote={We examine the Pb and Th abundances in 27 metal-poor stars (-3.1 < [Fe/H] < -1.4) whose very heavy metal (Z > 56) enrichment was produced only by the rapid (r-) nucleosynthesis process. New abundances are derived from HST/STIS, Keck/HIRES, and VLT/UVES spectra and combined with other measurements from the literature to form a more complete picture of nucleosynthesis of the heaviest elements produced in the r-process. In all cases, the abundance ratios among the rare earth elements and the 3rd r-process peak elements considered (La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solar system r-process abundance distribution. We compare the stellar observations with r-process calculations within the classical "waiting-point" approximation. In these computations a superposition of 15 weighted neutron-density components in the range 23 < log(n_n) < 30 is fit to the r-process abundance peaks to successfully reproduce both the stable solar system isotopic distribution and the stable heavy element abundance pattern between Ba and U in low-metallicity stars. Under these astrophysical conditions, which are typical of the "main" r-process, we find very good agreement between the stellar Pb r-process abundances and those predicted by our model. For stars with anomalously high Th/Eu ratios (the so-called actinide boost), our observations demonstrate that any nucleosynthetic deviations from the main r-process affect--at most--only the elements beyond the 3rd r-process peak, namely Pb, Th, and U. Our theoretical calculations also indicate that possible r-process abundance "losses" by nuclear fission are negligible for isotopes along the r-process path between Pb and the long-lived radioactive isotopes of Th and U.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Kratz, Karl-Ludwig and Frebel, Anna and Christlieb, Norbert and Pfeiffer, Bernd and Cowan, John J. and Sneden, Christopher}, year={2009}, month={Jun}, pages={1963–1980} }
 @article{roederer_2008, title={CHEMICAL INHOMOGENEITIES IN THE MILKY WAY STELLAR HALO}, volume={137}, ISSN={0004-6256 1538-3881}, url={http://dx.doi.org/10.1088/0004-6256/137/1/272}, DOI={10.1088/0004-6256/137/1/272}, abstractNote={We have compiled a sample of 699 stars from the recent literature with detailed chemical abundance information (spanning −4.2≲ [Fe/H] ≲+0.3), and we compute their space velocities and Galactic orbital parameters. We identify members of the inner and outer stellar halo populations in our sample based only on their kinematic properties and then compare the abundance ratios of these populations as a function of [Fe/H]. In the metallicity range where the two populations overlap (−2.5≲ [Fe/H] ≲−1.5), the mean [Mg/Fe] of the outer halo is lower than the inner halo by −0.1 dex. For [Ni/Fe] and [Ba/Fe], the star-to-star abundance scatter of the inner halo is consistently smaller than in the outer halo. The [Na/Fe], [Y/Fe], [Ca/Fe], and [Ti/Fe] ratios of both populations show similar means and levels of scatter. Our inner halo population is chemically homogeneous, suggesting that a significant fraction of the Milky Way stellar halo originated from a well-mixed interstellar medium. In contrast, our outer halo population is chemically diverse, suggesting that another significant fraction of the Milky Way stellar halo formed in remote regions where chemical enrichment was dominated by local supernova events. We find no abundance trends with maximum radial distance from the Galactic center or maximum vertical distance from the Galactic disk. We also find no common kinematic signature for groups of metal-poor stars with peculiar abundance patters, such as the α-poor stars or stars showing unique neutron-capture enrichment patterns. Several stars and dwarf spheroidal systems with unique abundance patterns spend the majority of their time in the distant regions of the Milky Way stellar halo, suggesting that the true outer halo of the Galaxy may have little resemblance to the local stellar halo.}, number={1}, journal={The Astronomical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U.}, year={2008}, month={Dec}, pages={272–295} }
 @inproceedings{roederer_lawler_sneden_cowan_sobeck_pilachowski_2008, series={Astronomical Society of the Pacific conference series}, title={Europium, Samarium, and Neodymium Isotopic Fractions in Metal-Poor Stars}, booktitle={New horizons in astronomy : Frank N. Bash Symposium 2007 : proceedings of a workshop held at the University of Texas, Austin, Texas, USA, 14-16 October 2007}, author={Roederer, I.U. and Lawler, J.E. and Sneden, C. and Cowan, J.J. and Sobeck, J. and Pilachowski, C.A.}, editor={Frebel, A.Editor}, year={2008}, pages={263}, collection={Astronomical Society of the Pacific conference series} }
 @article{roederer_lawler_sneden_cowan_sobeck_pilachowski_2008, title={Europium, Samarium, and Neodymium Isotopic Fractions in Metal‐Poor Stars}, volume={675}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1086/526452}, DOI={10.1086/526452}, abstractNote={We have derived isotopic fractions of europium, samarium, and neodymium in two metal-poor giants with differing neutron-capture nucleosynthetic histories. These isotopic fractions were measured from new high-resolution (R ∼ 120,000), high signal-to-noise ratio (S/N ~ 160-1000) spectra obtained with the 2d-coudé spectrograph of McDonald Observatory's 2.7 m Smith telescope. Synthetic spectra were generated using recent high-precision laboratory measurements of hyperfine and isotopic subcomponents of several transitions of these elements and matched quantitatively to the observed spectra. We interpret our isotopic fractions by the nucleosynthesis predictions of the stellar model, which reproduces s-process nucleosynthesis from the physical conditions expected in low-mass, thermally pulsing stars on the AGB, and the classical method, which approximates s-process nucleosynthesis by a steady neutron flux impinging on Fe-peak seed nuclei. Our Eu isotopic fraction in HD 175305 is consistent with an r-process origin by the classical method and is consistent with either an r- or an s-process origin by the stellar model. Our Sm isotopic fraction in HD 175305 suggests a predominantly r-process origin, and our Sm isotopic fraction in HD 196944 is consistent with an s-process origin. The Nd isotopic fractions, while consistent with either r-process or s-process origins, have very little ability to distinguish between any physical values for the isotopic fraction in either star. This study for the first time extends the n-capture origin of multiple rare earths in metal-poor stars from elemental abundances to the isotopic level, strengthening the r-process interpretation for HD 175305 and the s-process interpretation for HD 196944.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Lawler, James E. and Sneden, Christopher and Cowan, John J. and Sobeck, Jennifer S. and Pilachowski, Catherine A.}, year={2008}, month={Mar}, pages={723–745} }
 @misc{roederer_lawler_sneden_cowan_sobeck_pilachowski_2008, title={Europium, Samarium, and Neodymium Isotopic Fractions in Metal‐Poor Stars}, ISSN={0094-243X}, url={http://dx.doi.org/10.1063/1.2905533}, DOI={10.1063/1.2905533}, abstractNote={We have derived isotopic fractions of europium, samarium, and neodymium in two metal-poor giants with differing neutron-capture nucleosynthetic histories. These isotopic fractions were measured from new high-resolution (R ∼ 120,000), high signal-to-noise ratio (S/N ~ 160-1000) spectra obtained with the 2d-coudé spectrograph of McDonald Observatory’s 2.7 m Smith telescope. Synthetic spectra were generated using recent high-precision laboratory measurements of hyperfine and isotopic subcomponents of several transitions of these elements and matched quantitatively to the observed spectra. We interpret our isotopic fractions by the nucleosynthesis predictions of the stellar model, which reproduces s-process nucleosynthesis from the physical conditions expected in low-mass, thermally pulsing stars on the AGB, and the classical method, which approximates s-process nucleosynthesis by a steady neutron flux impinging on Fe-peak seed nuclei. Our Eu isotopic fraction in HD 175305 is consistent with an r-process origin by the classical method and is consistent with either an r- or an s-process origin by the stellar model. Our Sm isotopic fraction in HD 175305 suggests a predominantly r-process origin, and our Sm isotopic fraction in HD 196944 is consistent with an s-process origin. The Nd isotopic fractions, while consistent with either r-process or s-process origins, have very little ability to distinguish between any physical values for the isotopic fraction in either star. This study for the first time extends the n-capture origin of multiple rare earths in metal-poor stars from elemental abundances to the isotopic level, strengthening the r-process interpretation for HD 175305 and the s-process interpretation for HD 196944.}, journal={AIP Conference Proceedings}, publisher={American Institute of Physics}, author={Roederer, Ian U. and Lawler, James E. and Sneden, Christopher and Cowan, John J. and Sobeck, Jennifer S. and Pilachowski, Catherine A.}, year={2008} }
 @misc{roederer_sneden_lawler_sobeck_pilachowski_cowan_2008, title={Isotopic Abundances of Eu, Ba, and Sm in Metal-Poor Stars}, ISBN={9783540754848 9783540754855}, url={http://dx.doi.org/10.1007/978-3-540-75485-5_13}, DOI={10.1007/978-3-540-75485-5_13}, journal={Precision Spectroscopy in Astrophysics}, publisher={Springer Berlin Heidelberg}, author={Roederer, Ian U. and Sneden, Chris and Lawler, James E. and Sobeck, Jennifer S. and Pilachowski, Catherine A. and Cowan, John J.}, year={2008}, month={Feb}, pages={55–59} }
 @article{roederer_frebel_shetrone_allende prieto_rhee_gallino_bisterzo_sneden_beers_cowan_2008, title={The Hobby‐Eberly Telescope Chemical Abundances of Stars in the Halo (CASH) Project. I. The Lithium‐,<i>s</i>‐, and<i>r</i>‐enhanced Metal‐poor Giant HKII 17435−00532}, volume={679}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1086/587794}, DOI={10.1086/587794}, abstractNote={We present the first detailed abundance analysis of the metal-poor giant HKII 17435–00532. This star was observed as part of the University of Texas long-term project Chemical Abundances of Stars in the Halo (CASH). A spectrum was obtained with the High Resolution Spectrograph (HRS) on the Hobby-Eberly Telescope with a resolving power of R ∼ 15,000. Our analysis reveals that this star may be located on the red giant branch, red horizontal branch, or early asymptotic giant branch. We find that this metal-poor ([Fe/H] = − 2.2) star has an unusually high lithium abundance [log ε (Li) = + 2.1], mild carbon ([C/Fe] = + 0.7) and sodium ([Na/Fe] = + 0.6) enhancement, as well as enhancement of both s-process ([Ba/Fe] = + 0.8) and r-process ([Eu/Fe] = + 0.5) material. The high Li abundance can be explained by self-enrichment through extra mixing that connects the convective envelope with the outer regions of the H-burning shell. If so, HKII 17435–00532 is the most metal-poor star in which this short-lived phase of Li enrichment has been observed. The Na and n-capture enrichment can be explained by mass transfer from a companion that passed through the thermally pulsing AGB phase of evolution with only a small initial enrichment of r-process material present in the birth cloud. Despite the current nondetection of radial velocity variations (over ~180 days), it is possible that HKII 17435–00532 is in a long-period or highly inclined binary system, similar to other stars with similar n-capture enrichment patterns.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Roederer, Ian U. and Frebel, Anna and Shetrone, Matthew D. and Allende Prieto, Carlos and Rhee, Jaehyon and Gallino, Roberto and Bisterzo, Sara and Sneden, Christopher and Beers, Timothy C. and Cowan, John J.}, year={2008}, month={Jun}, pages={1549–1565} }
 @inproceedings{roederer_frebel_shetrone_prieto_rhee_gallino_bisterzo_sneden_beers_cowan_et al._2008, title={The Lithium-, r- and s-Enhanced Metal-Poor Giant HK-II 17435-00532}, ISSN={0094-243X}, url={http://dx.doi.org/10.1063/1.2916960}, DOI={10.1063/1.2916960}, abstractNote={We present the first detailed abundance analysis of the metal‐poor giant HK‐II 17435‐00532. This star was observed as part of the University of Texas Long‐Term Chemical Abundances of Stars in the Halo (CASH) Project. A spectrum was obtained with the High Resolution Spectrograph (HRS) on the Hobby‐Eberly Telescope with a resolving power of R∼15000. Our analysis reveals that this star may be located on the red giant branch, red horizontal branch, or early asymptotic giant branch. We find that this metal‐poor ([Fe/H] = −2.2) star has an unusually high lithium abundance (loge(Li) = +2.1), mild carbon ([C/Fe] = +0.7) and sodium ([Na/Fe] = +0.6) enhancement, as well as enhancement of both s‐process ([Ba/Fe] = +0.8) and r‐process ([Eu/Fe] = +0.5) material. The high Li abundance can be explained by self‐enrichment through extra mixing mechanisms that connect the convective envelope with the outer regions of the H‐burning shell. If so, HK‐II 17435‐00532 is the most metal‐poor starin which this short‐lived phase of...}, booktitle={AIP Conference Proceedings}, publisher={AIP}, author={Roederer, Ian U. and Frebel, Anna and Shetrone, Matthew and Prieto, Carlos Allende and Rhee, Jaehyon and Gallino, Roberto and Bisterzo, Sara and Sneden, Christopher and Beers, Timothy C. and Cowan, John J. and et al.}, year={2008} }
 @article{roederer_fredel_shetrone_allende prieto_rhee_gallino_bisterzo_sneden_beers_cowan_2007, title={The Hobby-Eberly Telescope "Chemical Abundances Of Stars In The Halo" (CASH) Project. I. The Lithium-, r-, and s-enhanced Metal-poor Giant HK-II 17435-00532}, volume={39}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U. and Fredel, A. and Shetrone, M. and Allende Prieto, C. and Rhee, J. and Gallino, R. and Bisterzo, S. and Sneden, C. and Beers, T.C. and Cowan, J.J.}, year={2007}, pages={959} }
 @article{roederer_sneden_lawler_sobeck_pilachowski_cowan_2006, title={Isotopic Abundances of Eu, Ba, and Sm in Metal-Poor Stars}, volume={38}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U. and Sneden, C. and Lawler, J.E. and Sobeck, J.S. and Pilachowski, C.A. and Cowan, J.J.}, year={2006}, pages={1138} }
 @article{mighell_roederer_2004, title={Flickering Red Giants in the Ursa Minor Dwarf Spheroidal Galaxy: Detection of Low-Amplitude Variability in Faint Red Giant Branch Stars on 10 Minute Timescales}, volume={617}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1086/426965}, DOI={10.1086/426965}, abstractNote={We have analyzed two epochs of Hubble Space Telescope Wide Field Planetary Camera 2 observations of the Ursa Minor dwarf spheroidal galaxy using the HSTphot photometric reduction package. We report the detection of nine faint (MV ≳ 0.0 mag) red giant variable stars that exhibit low-amplitude brightness fluctuations on 10 minute timescales with amplitudes ranging from 36 to 130 mmag. We have found variability in 14% of the red giants we have observed. If low-amplitude variability of red giants on 10 minute timescales can be verified and should their numbers prove to be at the 10% level or greater of all red giants in some ancient Population II stellar systems, then the observed color spread of the red giant branch of such systems would be broadened by flickering red giants in color-magnitude diagrams based on short (snapshot) observations of a single pair of 10 minute timescale observations in two different filters.}, number={1}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Mighell, Kenneth J. and Roederer, Ian U.}, year={2004}, month={Nov}, pages={L41–L44} }
 @article{roederer_deliyannis_platais_2003, title={WIYN Open Cluster Study: UBVRI CCD Photometry of the Hyades-aged Open Cluster NGC 6633}, volume={203}, journal={Bulletin of the American Astronomical Society}, author={Roederer, I.U. and Deliyannis, C.P. and Platais, I.}, year={2003}, pages={1408} }