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We study the formation of stars with varying amounts of heavy elements synthesized by the rapid neutron-capture process ($r$-process) based on our detailed cosmological zoom-in simulation of a Milky Way-like galaxy with an $N$-body/smoothed particle hydrodynamics code, ASURA. Most stars with no overabundance in $r$-process elements, as well as the strongly $r$-process enhanced $r$-II stars ([Eu/Fe] $>+0.7$), are formed in dwarf galaxies accreted by the Milky Way within the 6 Gyr after the Big Bang. In contrast, over half of the moderately enhanced $r$-I stars ($+0.3 <$ [Eu/Fe] $\leq +0.7$) are formed in the main in-situ disk after 6 Gyr. Our results suggest that the fraction of $r$-I and $r$-II stars formed in disrupted dwarf galaxies is larger the higher their [Eu/Fe] is. Accordingly, the most strongly enhanced $r$-III stars ([Eu/Fe] $> +2.0$) are formed in accreted components. These results suggest that non-$r$-process-enhanced stars and $r$-II stars are mainly formed in low-mass dwarf galaxies that hosted either none or a single neutron star merger, while the $r$-I stars tend to form in the well-mixed in-situ disk. We compare our findings with high-resolution spectroscopic observations of $r$-process-enhanced metal-poor stars in the halo and dwarf galaxies, including those collected by the R-Process Alliance. We conclude that observed [Eu/Fe] and [Eu/Mg] ratios can be employed in chemical tagging of the Milky Way's accretion history., Comment: 16 pages, 11 figures, submitted to ApJ

In this work, we present high-resolution (R~100,000), high signal-to-noise (S/N~800) spectroscopic observations for the well-known, bright, extremely metal-poor, carbon-enhanced star BD+44 493. We determined chemical abundances and upper limits for 17 elements from WIYN/NEID data, complemented with 11 abundances re-determined from Subaru and Hubble data, using the new, more accurate, stellar atmospheric parameters calculated in this work. Our analysis suggests that BD+44 493 is a low-mass (0.83Msun) old (12.1-13.2Gyr) second-generation star likely formed from a gas cloud enriched by a single metal-free 20.5Msun Population III star in the early Universe. With a disk-like orbit, BD+44 493 does not appear to be associated with any major merger event in the early history of the Milky Way. From the precision radial-velocity NEID measurements (median absolute deviation - MAD=16m/s), we were able to constrain companion planetary masses around BD+44 493 and rule out the presence of planets as small as msin(i)=2MJ out to periods of 100 days. This study opens a new avenue of exploration for the intersection between stellar archaeology and exoplanet science using NEID., Comment: 25 pages, 11 figures, accepted for publication on ApJ

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, 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 R-Process Alliance (RPA). We obtained high-resolution spectra for these objects with the HORuS spectrograph on the Gran Telescopio Canarias. We measure the abundances of light, alpha, Fe-peak, and neutron-capture elements. We report the discovery of five CEMP, 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 alpha-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., Comment: 34 pages, 16 figures, 7 tables; Accepted for publication in The Astrophysical Journal Supplement

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. 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. Assuming a canonical baryonic minihalo mass of 10^6 M_sun and a 1 percent metal retention rate, this star formed in a cloud of only ~ 600 M_sun., Comment: Accepted for publication in the Astrophysical Journal (23 pages, 9 figures)

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 6th 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., Comment: Accepted to MNRAS

We present spectroscopic data for 16369 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 multi-object, 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 1.1 km/s) effective temperature (234 K), (base10 logarithm of) surface gravity (0.52 dex in cgs units), [Fe/H] (0.38 dex) and [Mg/Fe] (0.24 dex) abundance ratios. The sample contains multi-epoch measurements for 3720 sources, with up to 15 epochs per source, enabling studies of intrinsic spectroscopic variability. The sample contains 6078 likely red giant stars (based on surface gravity), and 4494 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, 823 lie beyond twice the projected halflight radius of their host system, and 42 lie beyond 5 Rhalf., Comment: Published September 2023 in The Astrophysical Journal Supplement Series, processed spectra and catalogs publicly available at the Zenodo database doi:10.5281/zenodo.7837922

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 using nucleosynthesis models. We examine element abundances in a sample of stars that are enhanced in r-process elements. The abundances of elements Ru, Rh, Pd, and Ag (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., Comment: Authors' version of manuscript published in Science on December 07, 2023

We report on the chemo-dynamical analysis of SPLUS J142445.34-254247.1, an extremely metal-poor halo star enhanced in elements formed by the rapid neutron-capture process. This star was first selected as a metal-poor candidate from its narrow-band 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 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 logeps(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 was 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 Msun, and the heavy-element (Z>=38) abundance pattern can be reproduced by the yields from a neutron star merger (1.66Msun and 1.27Msun) 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., Comment: 26 pages, 11 figures, accepted for publication on ApJ

We present 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 alpha elements Mg, Si, Ca, and Ti ([alpha/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., Comment: Accepted for publication in the Astrophysical Journal (22 pages, 6 figures, 1 machine-readable table)

We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters (Gran 3/Patchick~125, Gran 4, Garro 01, LP 866) and two newly discovered open clusters (Gaia 9, Gaia 10) at low Galactic latitudes. We measure line-of-sight velocities and stellar parameters ([Fe/H], $\log{g}$, $T_{\rm eff}$, [Mg/Fe]) from high resolution spectroscopy centered 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]=-1.84) 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 disk 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 $R_{GC}\sim 18, 21.2~kpc$ and most metal-poor with [Fe/H]~-0.50,-0.46 for Gaia 9 and Gaia 10, respectively. LP 866 is a nearby, metal-rich open cluster ([Fe/H]$=+0.1$). The discovery and confirmation of multiple star clusters in the Galactic plane shows the power of {\it Gaia} astrometry and the star cluster census remains incomplete., Comment: 20 pages, 15 figures, accepted to MNRAS, associated data products available at https://doi.org/10.5281/zenodo.7809128

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 beryllium abundance survey. Using typically about 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 alpha elements Mg and Ca. We find good neutral/ion abundance agreement for the 6 elements that have detectable transitions of both species in our stars in the 3100-5800 Angstrom 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., Comment: ApJ, in press

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 $\lambda3859$ \r{A}. 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 $\lambda4050$ \r{A} and $\lambda4090$ \r{A}, in addition to the canonical $\lambda3859$ \r{A} line. We find that the U II lines at $\lambda4050$ \r{A} and $\lambda4090$ \r{A} are reliable and render U abundances in agreement with the $\lambda3859$ U abundance, for all 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., Comment: Resubmitted to ApJ

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., Comment: 16 pages, 3 figures, including 4 appendices. Published in the Astrophysical Journal

With the most trans-iron elements detected of any star outside the Solar System, HD 222925 represents the most complete chemical inventory among r-process-enhanced, metal-poor stars. While the abundance pattern of the heaviest elements identified in HD 222925 agrees with the scaled Solar r-process residuals, as is characteristic of its r-process-enhanced classification, the newly measured lighter r-process elements display marked differences from their Solar counterparts. In this work, we explore which single astrophysical site (if any) produced the entire range of elements ($34\leq Z\leq 90$) present in HD 222925. We find that the abundance pattern of lighter r-process elements newly identified in HD 222925 presents a challenge for our existing nucleosynthesis models to reproduce. The most likely astrophysical explanation for the elemental pattern of HD 222925 is that its light r-process elements were created in rapidly expanding ejecta (e.g., from shocked, dynamical ejecta of compact object merger binaries). However, we find that the light r-process-element pattern can also be successfully reproduced by employing different nuclear mass models, indicating a need for a fresh investigation of nuclear input data for elements with $46\lesssim Z\lesssim 52$ by experimental methods. Either way, the new elemental abundance pattern of HD 222925 -- particularly 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 for the r-process at low metallicity., Comment: 14 pages, 9 figures

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 ultra high 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_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., Comment: 25 pages, 4 tables, 10 figures, Accepted for publication in ApJ

Orbital characteristics based on Gaia Early Data Release 3 astrometric parameters are analyzed for ${\sim} 1700$ $r$-process-enhanced (RPE; [Eu/Fe] $> +0.3$) metal-poor stars ([Fe/H] $\leq -0.8$) 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 $36$ Chemo-Dynamically Tagged Groups (CDTGs) containing between $5$ 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 re-identified. Associations with MW globular clusters are determined for $7$ 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 the Yoon-Beers $A$(C)$_{c}$ vs. [Fe/H] Diagram. Our results confirm previous dynamical analyses that showed RPE stars in CDTGs share common chemical histories, influenced by their birth environments., Comment: accepted by ApJ. arXiv admin note: substantial text overlap with arXiv:2201.08337, arXiv:2109.08600

We present a high-resolution ($R\sim35,000$), 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), [Fe/H] = $-$3.7 (spectroscopic NLTE stellar parameters). We measured a carbon-to-iron ratio of $\mbox{[C/Fe]}= 0.38$ from the CH G-band. J1808$-$5104 is thus not carbon-enhanced, contrary to many other stars with similarly low iron abundances. We also determine, for the first time, a barium abundance ($\mbox{[Ba/Fe]} =-0.78$), and obtain a significantly reduced upper limit for the nitrogen abundance ([N/Fe]$ < - 0.2$). J1808$-$5104 has low ratio of $\mbox{[Sr/Ba]}=-0.17$, which is consistent with that of stars in ultra-faint dwarf galaxies. We also fit the abundance pattern of J1808$-$5104 with nucleosynthesis yields from a grid of Population\,III supernova models. There is a good fit to the abundance pattern which suggests J1808$-$5104 originated from gas enriched by a single massive supernova with a high explosion energy of E $=10\times10^{51}$\,erg and a progenitor stellar mass of M$=29.5$\,M$_{\odot}$. Interestingly, J1808$-$5104 is a member of the Galactic thin disk, as confirmed by our detailed kinematic analysis and calculated stellar actions and velocities. Finally, we also established the orbital history of J1808$-$5104 using our time-dependent Galactic potential the \texttt{ORIENT}. J1808$-$5104 appears to have a stable quasi-circular orbit and been largely confined to the thin disk. This unique orbital history, the star's very old age ($\sim13.5$\,Gyr), and the low [C/Fe] and [Sr/Ba] ratios suggest that J1808$-$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 disk.

$R$-process enhanced stars with [Eu/Fe]$\geq+0.7$ (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 the Gaia EDR3 data. Since the observational uncertainty is not negligible, we applied a newly-developed {\it 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., Comment: Main text 21 pages, References 3 pages, Appendix 16 pages. 11 figures. 3 tables. ApJ accepted. (See arXiv:2204.08205 for the algorithm of our method.)

The ultra-faint 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 $72^{+10}_{-12}$% of the stars are r-process-enhanced, with a mean $\left\langle\mbox{[Ba/H]}\right\rangle=-1.68~\pm~0.07$ and unresolved intrinsic dispersion $\sigma_{\rm [Ba/H]} < 0.20$. 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 ultra-faint 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 ${\approx}70$% of Ret~II stars formed after reionization., Comment: 37 pages, 12 figures, 10 tables, accepted to AJ

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, totaling 63 metals detected and 7 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., Comment: Accepted for publication in the Astrophysical Journal Supplement Series (40 pages, 10 figures; includes 43 appendices with detailed discussion of line detections and atomic data). Version 2 corrects a typo in the 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 multi-year radial velocity survey of the cluster cores. Six of these systems are double-lined spectroscopic binaries (SB2s). 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 radial velocity prior on all cluster members; this significantly improves our orbital constraints. Two of our systems have periods between 5 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 lightcurves 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., Comment: 38 pages, 47 figures (incl. 37 Appendix figures), 4 tables. AJ accepted

Outflows from super-massive 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 6.5m 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 -6500 km/s relative to the H{\alpha} 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{\alpha}, 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., Comment: Accepted for publication in the MNRAS. 15 pages, 7 figures, 3 tables

We report on the discovery of SPLUS J210428.01-004934.2, an ultra metal-poor (UMP) star first identified from the narrow-band 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 narrow-band 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 bonafide second-generation star, formed from a gas cloud polluted by a single metal-free ~30Mo star. This discovery opens the possibility of finding additional UMP stars directly from narrow-band photometric surveys, a potentially powerful method to help complete the inventory of such peculiar objects in our Galaxy., Comment: 10 pages, 3 figures, accepted for publication on ApJ Letters

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., Comment: 3 pages, 1 interactive figure (http://vmplacco.github.io/files/linemake_ptable.html)

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 ($M/L_V$) ratios over a wide range in age and metallicity. We obtained 3137 high-resolution stellar spectra with M2FS on the \textit{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 $M/L_V$ estimates for each cluster. Mean metallicities for the clusters were also obtained and tied to a scale based on calcium IR triplet metallicites. We present trends of the cluster $M/L_V$ 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 $M/L_V$ values, significantly increasing the discrepancy with our observations., Comment: 33 pages, 23 figures, 10 tables. Accepted for publication in MNRAS

We report the detection of an Al II line at 2669.155 Angstroms 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 (sigma = 0.22) for these 11 stars spanning -3.9 < [Fe/H] < -1.3, or [Al/Fe] = -0.10 +/- 0.04 (sigma = 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 Angstroms. 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., Comment: Accepted for publication in the Astrophysical Journal (13 pages, 5 figures, 6 tables)

We present high-resolution Magellan/MIKE spectra of 22 bright ($9

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 ten-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 Chemo-Dynamically 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 ultra-faint 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., Comment: To be published in the Astrophysical Journal

We report the first detection of the Pb II line at 2203.534 Angstroms 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 Pb-208 is the dominant Pb isotope in these two stars. The log epsilon(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., Comment: Accepted for publication in the Astrophysical Journal Letters (7 pages, 2 figures)

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 Coud\'{e} 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 1 to 4 lines of V I, and from 2 to 7 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 \pm 0.01 (\sigma = 0.16)$ from 128 stars with $\geq$ 2 V I lines detected, [V II/Fe II] $= +0.13 \pm 0.01 (\sigma = 0.16)$ from 220 stars with $\geq$ 2 V II lines detected, and [V II/V I] $= +0.25 \pm 0.01 (\sigma = 0.15)$ from 119 stars. We suspect this offset is due to non-LTE effects, 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., Comment: 19 pages, 15 figures, 4 tables, accepted for publication in ApJ

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 ($R \sim 30,000$) spectra collected with the du Pont 2.5m Telescope. In this data release, we propose a new delineation between the $r$-I and $r$-II stellar classes at $\mathrm{[Eu/Fe]} = +0.7$, instead of the empirically chosen $\mathrm{[Eu/Fe]} = +1.0$ 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 analysis of the possible progenitors of these two classes of stars and 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 three re-identified), and 7 new (plus one 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., Comment: 17 pages, 7 figures, 6 tables; accepted to ApJS

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 over-abundance 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 (+0.3<=[Eu/Fe]<=+1.0) r-I and strongly r-process enhanced ([Eu/Fe]>+1.0) r-II stars to include an r-III class, for r-process stars such as J1521-3538, with [Eu/Fe]>+2.0 and [Ba/Eu]<-0.5, or >100 times the solar ratio of europium to iron. Using cosmochronometry, we estimate J1521-3538 to be 12.5+-5 Gyr and 8.9+-5 Gyr old, using two different sets of initial production ratios. These ages are based on measurements of the Th line at 4019 A 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., Comment: 24 pages, includes a long table, 4 figures

Extensive progress has been recently made into 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 provides key evidence into 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 Non-Local 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 6.5m 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] $\le +1.0$) 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 $\alpha$ 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., Comment: 17 pages, 9 figures, 6 tables, Accepted for publication in ApJ

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 [Fe/H]=-3.57, this is one of the lowest metallicity stars 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 suggest 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 fast 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., Comment: Accepted for publication in ApJ

We have obtained new detailed abundances of the Fe-group elements Sc through Zn (Z=21-30) in three very metal-poor ([Fe/H] $\approx -3$) stars: BD 03 740, BD -13 3442 and CD -33 1173. High-resolution ultraviolet HST/STIS spectra in the wavelength range 2300-3050\AA\ were gathered, and complemented by an assortment of optical echelle spectra. The analysis featured recent laboratory atomic data for 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 03 740, BD -13 3442 and CD -33 1173 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 [Cri/Fe]=+0.01) and ionized species (mean of [Crii/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/H]=-2.53), while no such enhancement is seen for the ionized species (mean of [Coii/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., Comment: 30 pages, 11 figures. To appear in the Astrophysical Journal

As an introduction of a kinematic survey of Magellanic Cloud (MC) star clusters, we report on the dynamical masses and mass-to-light ($M/L$) ratios of NGC 419 (SMC) and NGC 1846 (LMC). 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 ($P_{M}\geq 50$%) 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-fit results show that NGC 419 has a projected central velocity dispersion of $2.44^{+0.37}_{-0.21}\ {\rm km\,s^{-1}}$, corresponding to a total mass of $7.6^{+2.5}_{-1.3}\times10^4\ {\rm M}_{\odot}$ and $V$-band $M/L$ ratio of $0.22^{+0.08}_{-0.05}$ in solar units. For NGC 1846, the corresponding results are $2.04^{+0.28}_{-0.24}\ {\rm km\,s^{-1}}$, $5.4^{+1.5}_{-1.4}\times10^4\ {\rm M}_{\odot}$ and $0.32^{+0.11}_{-0.11}$. The mean metallicities of NGC 419 and NGC 1846 are found to be $\rm [Fe/H]=-0.84\pm0.19$ and $-0.70\pm0.08$, 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., Comment: 24 pages, 15 figures, 10 tables, accepted for publication in MNRAS

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 1.2 sigma) 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 alpha elements Mg, Si, and Ca ([alpha/Fe] = +0.32 +/- 0.06), and 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 (log epsilon (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 globular cluster. 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 globular cluster, it would be the most metal-poor globular cluster known., Comment: Accepted for publication in the Astrophysical Journal (17 pages, 4 figures, 3 tables)

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, HE1327-2326, with [Fe/H](NLTE) = -5.2. We determine [Zn/Fe] = 0.80$\pm$0.25 from a UV Zn I line at 2138 detected at $3.4\sigma$. Yields of a 25M$_{\odot}$ aspherical supernova model with artificially modified densities exploding with E = 5x10$^{51}$ ergs best match the entire abundance pattern of HE1327-2326. Such high-entropy hypernova explosions are expected to produce bipolar outfows 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 HE1327-2326 might have formed in such externally enriched environments., Comment: 11 pages, 4 figures, accepted for publication in the Astrophysical Journal

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, alpha, and Fe-peak element. The low [alpha/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., Comment: Accepted for publication in the Astrophysical Journal

We present results from an observing campaign to identify low-metallicity stars in the Best & Brightest Survey. From medium-resolution (R ~ 1, 200 - 2, 000) spectroscopy of 857 candidates, we estimate the stellar atmospheric parameters (Teff, log g, and [Fe/H]), as well as carbon and alpha-element abundances. We find that 69% of the observed stars have [Fe/H] <= -1.0, 39% have [Fe/H] <= -2.0, and 2% have [Fe/H] <= -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., Comment: 57 pages, accepted for publication in ApJ

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 ThetaE=36'' at z = 9 and a mass projected within 200 kpc of M(200 kpc) = 1.78+0.01-0.03x10**14Msol. 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 (MAST)., Comment: 20 pages, 8 figures. Accepted to ApJ

We report that LAMOST-HVS1 is a massive hyper-runaway subgiant star with mass of 8.3 Msun and super-Solar metallicity, ejected from the inner stellar disk of the Milky Way $\sim$ 33 Myr ago with the intrinsic ejection velocity of $568^{+19}_{-17}$ km/s (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 the binary companion. Rather, it was probably ejected by a 3- or 4-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 with mass of $\gtrsim 10^4$ Msun. The ejection agent that took part in the ejection of LAMOST-HVS1 may well be an intermediate mass black hole ($\gtrsim$ 100 Msun), a very massive star ($\gtrsim$ 100 Msun), or multiple ordinary massive stars ($\gtrsim$ 30 Msun). 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 young massive cluster near the Norma spiral arm., Comment: Minor revision after referee's comments (conclusion unchanged). ApJ submitted. 23 pages. 7 figures. 2 tables. Predicted location of IMBH (or the undiscovered young massive cluster) in the Milky Way is shown in Fig 7. High-resolution spectra (Magellan/MIKE) can be distributed upon request. Comments welcome

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 <3D> 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, 3 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, alpha, and Fe-peak elements match those of typical Milky Way halo stars, except for one r-I star which 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 which are consistent with membership in the Milky Way halo. Intriguingly, all r-II and the majority of r-I stars have retrograde orbits, which may indicate an accretion origin., Comment: Accepted for publication in the Astrophysical Journal

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., Comment: Proceedings for the FRIB Theory Alliance workshop "FRIB and the GW170817 kilonova", held 16-27 July 2018 at the Facility for Rare Isotope Beams, Michigan State University, East Lansing, MI USA; 44 pages, 14 figures

We present 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 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., Comment: Accepted for publication in the Astronomical Journal (19 pages, 10 figures, 6 tables)

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., Comment: Accepted for publication in the Astrophysical Journal (17 pages, 4 figures, 3 tables)

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 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., Comment: 30 pages, includes a long table, 5 figures

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 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., Comment: 8 pages, 3 figures, 2 tables