arXiv subjects

Sarah Loebman

Publications and source records attributed to Sarah Loebman.

At least 19 recordsLinked to original sources

arXiv2026

OCCAM X. Neutron Capture Abundances with Keck/HIRES & Magellan/MIKE

The chemistry of stars provides powerful insight into the history of the Milky Way. With multiple large-sky spectroscopic surveys that are currently available, using chemistry as a means to study the evolution and history of the Milky Way has flourished. Open clusters have long been used as landmarks to calibrate different age dating methods (e.g., gyrochronology and asteroseismology). In this work, we utilize the SDSS-IV/APOGEE-based Open Cluster Chemical Abundances and Mapping (OCCAM) survey as our foundation for new optical observations; enabling us to characterize neutron-capture abundances for known cluster members. For 56 stars in 18 open clusters, we collected high-resolution (R > 50,000), high-S/N (>75 at 5500A), spectra from Keck I and Magellan Baade telescopes. With these data, we derive abundances for 23 elements using BACCHUS, including 7 neutron capture abundances not measurable by APOGEE. Finally, we characterize the radial distribution of these neutron-capture elements in the Milky Way. We find that the second-peak s-process and r-process abundances exhibit relatively flat gradients in the Milky Way. Although not as distinct, the first-peak s-process abundances also have slopes which are shallower than the alpha and iron-peak elements. The differences in the neutron-capture gradients from the lighter elements not just indicates the sources producing these elements are fundamentally different, but that the timescales on which they are produced also differ (especially for the r-process). Moreover, a metallicity dependence of the AGB stars responsible for producing the heaviest s-process abundances may be necessary to consider in Galactic evolution models.

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arXiv2026

Hot or Cold? Radial Redistribution of Stars in FIRE Simulations of Milky Way-Mass Galaxies and the Asymmetry of Inward versus Outward Migrators

Stars can radially redistribute (migrate) within galactic disks. The degree to which this occurs as dynamically `cold' (preserves orbital eccentricity) or `hot' (increases eccentricity) remains debated. Many models presume that radial redistribution occurs primarily via cold torquing, resulting in changes in angular momentum without dynamical heating. We test the net dynamical heating associated with redistribution over stellar lifetimes using the FIRE cosmological zoom-in simulations of 12 Milky Way-mass galaxies. We select star particles today that underwent significant changes in orbital angular momentum, j_phi, since birth. We investigate net changes in their orbital eccentricity, e, and we quantify the `cold-torqued' fraction of star particles with |Delta j_phi/j_phi,birth| > 0.2 that preserved eccentricity (|Delta e| < 0.1) since birth. The direction of radial redistribution is most critical: outward-migrating stars experienced smaller net changes in eccentricity, whereas inward-migrating stars almost always heat since birth. For stars born on near-circular orbits (e_birth < 0.2), the cold-torqued fraction decreases rapidly with age today and is generally < 50% at ages >~2 Gyr. Stars born on moderately eccentric orbits (e_birth ~ 0.4) are the most likely to preserve their birth eccentricity. However, the cold-torqued fraction is higher in earlier-forming and/or dynamically-colder disks. Significantly, we identify a population of stars that dynamically `cooled', decreasing in eccentricity since birth: this is the primary way that stars end up on near-circular orbits today. Overall, a star's migration direction, its e_birth, and its age primarily determine whether it was dynamically heated, cooled, or unchanged. In general, radial redistribution in FIRE is typically not cold between birth and today.

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arXiv2026

The Chemical Homogeneity of Single-Lined Spectroscopic Binaries in Open Clusters

Using SDSS-V DR19 Milky Way Mapper APOGEE data, we measure the impact that close binarity has on surface chemistry across the Hertzsprung-Russell diagram in a broad set of abundances by studying single-lined spectroscopic binaries (SB1s) in open clusters. We derive binary membership and orbital parameters for 103 SB1s by analysing APOGEE radial velocities with The Joker and UltraNest. We perform a detailed abundance analysis with BACCHUS to derive abundances in fourteen chemical species: Si, Fe, C, N, O, Na, Mg, Al, Ca, Ti, Cr, Ni, Ce, and Nd. Leveraging the assumptions of chemical homogeneity in open clusters, we compare the surface abundances of SB1s to non-binary stars at similar evolutionary states. We find that a subset of binaries with significant UV excess have a $\Delta$[C/N] that is 0.2--0.5 dex higher than expected, resulting in overestimated [C/N]-based ages for those stars. This points to pollution from an evolved companion and has implications for [C/N]-based age studies of the broader Milky Way. At the population level, we find that SB1s in our sample can be treated as statistically chemically homogeneous with their single-star counterparts, and we find no connection between orbital separation and chemical enrichment or depletion. We show that at separations up to ~5 pc, co-eval stars can be considered chemically homogeneous with one another within current abundance precisions, regardless of multiplicity.

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arXiv2026

The origin of strong $\alpha$-element bimodalities in FIRE simulations of Milky Way-mass galaxies

One of the Milky Way's characteristic features is a strongly bimodal distribution of $\alpha$-process elements, such as Mg, at fixed [Fe/H] in stellar abundances. We examine patterns in [Mg/Fe] versus [Fe/H] in FIRE-2 simulations of Milky Way-mass galaxies. Out of 16 galaxies, 4 are capable of producing a strongly bimodal distribution. In all four galaxies, the high-$\alpha$ population corresponds to an older, radially-compact, thick disk, and the low-$\alpha$ population corresponds to a younger, radially-extended, thin disk, similar to the MW.The transition from high- to low-$\alpha$ took $0.3-1.2\Gyr$ and began $5.5-6.5\Gyr$ ago. [Mg/Fe] decreased at relatively fixed [Fe/H], both in the galaxy overall and at fixed radii: Fe enrichment nearly balanced gas accretion (and therefore dilution), but Mg enrichment was weaker. Importantly, this transition occurred during a period of relatively low gas fraction ($5-15\%$), immediately after a rapid decline in star formation (halving within a few hundred Myr), which caused an increase in Fe-producing white-dwarf supernovae relative to Mg-producing core-collapse supernovae. Only one case coincided with a major merger coalescence. We find similar trends in measuring stars by their current radius and by their birth radius, therefore, radial redistribution did not play a dominant role in the formation of a bimodality or its spatial dependence today. Overall, in FIRE-2, strong $\alpha$-element bimodalities are relatively uncommon ($\sim25\%$), often not associated with a major merger, and arise primarily from a rapid decline in star formation during relatively low gas fraction.

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arXiv2025

Only the Special Survive: Evolution of Long-Lived Star Clusters in Galaxy Simulations

In this work, we aim to answer one crucial question behind the discrepancy between chemical trends of field stars and clusters in the Galactic disk: is the chemical gradient mismatch driven by cluster migration and differential survivability as a function of galactic location? To answer this question, we explored the evolution of long-lived (> 1 Gyr) star clusters in Milky Way-galaxy simulations. In particular, we investigated why some star clusters remain bound over billions of years. We have traced the unique trajectories for a sample of open clusters around two FIRE galaxies throughout cosmic time. Additionally, we characterized the small-scale environment surrounding these clusters over their orbital history. We see that clusters across both FIRE galaxies spend the majority of their lives in under-dense regions of gas, except for brief passages where they interact with gas clouds, causing their orbits to be altered.

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arXiv2025

Beyond OCCAM: Measuring Optical Neutron Capture Abundances of Open Cluster Stars

Open clusters have long been used to determine ages of stars, as well as calibrate stellar evolution models and other methods of age-dating stellar groups, e.g., gyrochronology, asteroseismology, and chemical clocks. In this work, we have obtained new high-resolution (R $\ge$ 50,000), high-S/N, optical data for 3+ stellar members in open clusters, using Keck/HIRES, with membership derived from the Open Cluster Chemical Abundances and Mapping (OCCAM) survey. From these new Keck/HIRES data, we have derived neutron capture abundances for stars in seven distant outer Galaxy open cluster

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arXiv2025

Second public data release of the FIRE-2 cosmological zoom-in simulations of galaxy formation

We describe the second data release (DR2) of the FIRE-2 cosmological zoom-in simulations of galaxy formation, from the Feedback In Realistic Environments (FIRE) project, available at http://flathub.flatironinstitute.org/fire. DR2 includes all snapshots for most simulations, starting at z ~ 99, with all snapshot time spacings <~ 25 Myr. The Core suite -- comprising 14 Milky Way-mass galaxies, 5 SMC/LMC-mass galaxies, and 4 lower-mass galaxies -- includes 601 snapshots to z = 0. For the Core suite, we also release resimulations with physics variations: (1) dark-matter-only versions; (2) a modified ultraviolet background with later reionization at z = 7.8; (3) magnetohydrodynamics, anisotropic conduction, and viscosity in gas; and (4) a model for cosmic-ray injection, transport, and feedback (assuming a constant diffusion coefficient). The Massive Halo suite now includes 8 massive galaxies with 278 snapshots to z = 1. The High Redshift suite includes 34 simulations: in addition to the 22 simulations run to z = 5, we now include 12 additional simulations run to z = 7 and z = 9. We also release 4 dark-matter-only cosmological boxes used to generate zoom-in initial conditions for many FIRE simulations. Most simulations include catalogs of (sub)halos and galaxies at all available snapshots, and most Core simulations to z = 0 include full halo merger trees.

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arXiv2025

Stellar Velocity Dispersion versus Age: Consistency across Observations and Simulations, with the Milky Way as an Outlier

Within disk galaxies, the velocity dispersion, $\sigma$, of stars increases with age, $\tau$, as measured in the Milky Way (MW) and nearby galaxies. This relation provides a key window into galactic formation history, tracing both the kinematics of stars at birth and the dynamical heating of stars after birth. We compile and compare observational measurements of the MW, M31, M33, and 16 galaxies from the PHANGS survey. The MW exhibits significantly colder stellar kinematics, with 2-3 times lower $\sigma(\tau)/v_{\phi,0}$ at a given age, than all but one other observed galaxy. Therefore, the MW is a kinematic outlier. To assess how measurement effects influence $\sigma(\tau)$, we analyze the FIRE-2 cosmological simulations, quantifying the impact of uncertainties in stellar age, aperture size, galactocentric radius, and galaxy inclination. Aperture size and galactocentric radius affect $\sigma(\tau)$ by up to a factor of $\approx2$ for stars younger than 100 Myr, with milder effects on older stars. Age uncertainties up to 40\% change the \textit{value} of $\sigma(\tau)$ at a given age by $\lesssim20\%$ but can reshape the relation with age and erase merger signatures. We compare $\sigma(\tau)/v_{\phi,0}$ in FIRE-2 simulations with observations. FIRE-2 agrees well with M31 and M33 at all measured ages, and with PHANGS for stars older than $\approx500$ Myr. The average $\sigma(\tau)/v_{\phi,0}$ in FIRE-2 is about two times higher than the MW at most ages, but the youngest stars show better agreement. The velocity ratios ($\sigma_{\phi}/\sigma_{R}$, $\sigma_{Z}/\sigma_{\phi}$, $\sigma_{Z}/\sigma_{R}$) in FIRE-2 broadly agree with the MW. We conclude that $\sigma(\tau)$ in FIRE-2, and most cosmological zoom-in simulations, reasonably matches observed nearby galaxies, but matching the MW is rare, because it is a kinematic outlier.

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arXiv2024

Spectacular nucleosynthesis from early massive stars

Stars formed with initial mass over 50 Msun are very rare today, but they are thought to be more common in the early universe. The fates of those early, metal-poor, massive stars are highly uncertain. Most are expected to directly collapse to black holes, while some may explode as a result of rotationally powered engines or the pair-creation instability. We present the chemical abundances of J0931+0038, a nearby low-mass star identified in early followup of SDSS-V Milky Way Mapper, which preserves the signature of unusual nucleosynthesis from a massive star in the early universe. J0931+0038 has relatively high metallicity ([Fe/H] = -1.76 +/- 0.13) but an extreme odd-even abundance pattern, with some of the lowest known abundance ratios of [N/Fe], [Na/Fe], [K/Fe], [Sc/Fe], and [Ba/Fe]. The implication is that a majority of its metals originated in a single extremely metal-poor nucleosynthetic source. An extensive search through nucleosynthesis predictions finds a clear preference for progenitors with initial mass > 50 Msun, making J0931+0038 one of the first observational constraints on nucleosynthesis in this mass range. However the full abundance pattern is not matched by any models in the literature. J0931+0038 thus presents a challenge for the next generation of nucleosynthesis models and motivates study of high-mass progenitor stars impacted by convection, rotation, jets, and/or binary companions. Though rare, more examples of unusual early nucleosynthesis in metal-poor stars should be found in upcoming large spectroscopic surveys.

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arXiv2022

Public data release of the FIRE-2 cosmological zoom-in simulations of galaxy formation

We describe a public data release of the FIRE-2 cosmological zoom-in simulations of galaxy formation, available at http://flathub.flatironinstitute.org/fire, from the Feedback In Realistic Environments (FIRE) project. FIRE-2 simulations achieve parsec-scale resolution to explicitly model the multi-phase interstellar medium while implementing direct models for stellar evolution and feedback, including stellar winds, core-collapse and Ia supernovae, radiation pressure, photoionization, and photoelectric heating. We release complete snapshots from 3 suites of simulations. The first comprises 20 simulations that zoom in on 14 Milky Way-mass galaxies, 5 SMC/LMC-mass galaxies, and 4 lower-mass galaxies including 1 ultra-faint; we release 39 snapshots across z = 0 - 10. The second comprises 4 massive galaxies, with 19 snapshots across z = 1 - 10. Finally, a high-redshift suite comprises 22 simulations, with 11 snapshots across z = 5 - 10. Each simulation also includes dozens of resolved lower-mass (satellite) galaxies in its zoom-in region. Snapshots include all stored properties for all dark matter, gas, and star particles, including 11 elemental abundances for stars and gas, and formation times (ages) of star particles. We also release accompanying (sub)halo catalogs, which include galaxy properties and member star particles. For the simulations to z = 0, including all Milky Way-mass galaxies, we release the formation coordinates and an "ex-situ" flag for all star particles, pointers to track particles across snapshots, catalogs of stellar streams, and multipole basis expansions for the halo mass distributions. We describe publicly available python packages for reading and analyzing these simulations.

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arXiv2021

New families in our Solar neighborhood: applying Gaussian Mixture models for objective classification of structures in the Milky Way and in simulations

The standard picture of galaxy formation motivates the decomposition of the Milky Way into 3--4 stellar populations with distinct kinematic and elemental abundance distributions: the thin disk, thick disk, bulge, and stellar halo. To test this idea, we construct a Gaussian mixture model (GMM) for both simulated and observed stars in the Solar neighborhood, using measured velocities and iron abundances (i.e., an augmented Toomre diagram) as the distributions to be decomposed. We compare results for the Gaia-APOGEE DR16 crossmatch catalog of the Solar neighborhood with those from a suite of synthetic Gaia-APOGEE crossmatches constructed from FIRE-2 cosmological simulations of Milky Way-mass galaxies. We find that in both the synthetic and real data, the best-fit GMM uses five independent components, some of whose properties resemble the standard populations predicted by galaxy formation theory. Two components can be identified unambiguously as the thin disk and another as the halo. However, instead of a single counterpart to the thick disk, there are three intermediate components with different age and alpha abundance distributions (although these data are not used to construct the model). We use decompositions of the synthetic data to show that the classified components indeed correspond to stars with different origins. By analogy with the simulated data, we show that our mixture model of the real Gaia-APOGEE crossmatch distinguishes the following components: (1) a classic thin disk of young stars on circular orbits (46%), (2) thin disk stars heated by interactions with satellites (22%), (3, 4) two components representing the velocity asymmetry of the alpha-enhanced thick disk (27%), and (5) a stellar halo consistent with early, massive accretion (4%).

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arXiv2020

One-Two Quench: A Double Minor Merger Scenario

Using the N-body+Smoothed particle hydrodynamics code, ChaNGa, we identify two merger-driven processes\textemdash disk disruption and supermassive black hole (SMBH) feedback\textemdash which work together to quench L$^*$ galaxies for over 7 Gyr. Specifically, we examine the cessation of star formation in a simulated Milky Way (MW) analog, driven by an interaction with two minor satellites. Both interactions occur within $\sim$100 Myr of each other, and the satellites both have masses 5 to 20 times smaller than that of their MW-like host galaxy. Using the genetic modification process of \cite{Roth2016}, we generate a set of four zoom-in, MW-mass galaxies all of which exhibit unique star formation histories due to small changes to their assembly histories. In two of these four cases, the galaxy is quenched by $z = 1$. Because these are controlled modifications, we are able to isolate the effects of two closely-spaced minor merger events, the relative timing of which determines whether the MW-mass main galaxy quenches. This one-two punch works to: 1. fuel the primary halo's supermassive black hole (SMBH) at its peak accretion rate; and 2. disrupt the cold, gaseous disk of the host galaxy. The end result is that feedback from the SMBH thoroughly and abruptly ends the galaxy's star formation by $z\approx1$. We search for and find a similar quenching event in {\sc Romulus25}, a hydrodynamical $(25\,\mathrm{Mpc})^3$ volume simulation, demonstrating that the mechanism is common enough to occur even in a small sample of MW-mass quenched galaxies at $z=0$.

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arXiv2020

Reproducing the CO-to-H$_2$ conversion factor in cosmological simulations of Milky Way-mass galaxies

We present models of CO(1-0) emission from Milky Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1-0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which in our models sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H$_2$ abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of $\sim 3$ pc in cold gas ($T < 300$ K) for both CO and H$_{2}$, is able to reproduce both the observed CO(1-0) luminosity and inferred CO-to-H$_{2}$ conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model which controls the amount of radiation that penetrates giant molecular clouds.

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arXiv2019

Evolution of giant molecular clouds across cosmic time

Giant molecular clouds (GMCs) are well-studied in the local Universe, however, exactly how their properties vary during galaxy evolution is poorly understood due to challenging resolution requirements, both observational and computational. We present the first time-dependent analysis of giant molecular clouds in a Milky Way-like galaxy and an LMC-like dwarf galaxy of the FIRE-2 (Feedback In Realistic Environments) simulation suite, which have sufficient resolution to predict the bulk properties of GMCs in cosmological galaxy formation self-consistently. We show explicitly that the majority of star formation outside the galactic center occurs within self-gravitating gas structures that have properties consistent with observed bound GMCs. We find that the typical cloud bulk properties such as mass and surface density do not vary more than a factor of 2 in any systematic way after the first Gyr of cosmic evolution within a given galaxy from its progenitor. While the median properties are constant, the tails of the distributions can briefly undergo drastic changes, which can produce very massive and dense self-gravitating gas clouds. Once the galaxy forms, we identify only two systematic trends in bulk properties over cosmic time: a steady increase in metallicity produced by previous stellar populations and a weak decrease in bulk cloud temperatures. With the exception of metallicity we find no significant differences in cloud properties between the Milky Way-like and dwarf galaxies. These results have important implications for cosmological star and star cluster formation and put especially strong constraints on theories relating the stellar initial mass function to cloud properties.

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arXiv2019

When Cold Radial Migration is Hot: Constraints from Resonant Overlap

It is widely accepted that stars in a spiral disk, like the Milky Way's, can radially migrate on order a scale length over the disk's lifetime. With the exception of cold torquing, also known as "churning," processes that contribute to the radial migration of stars are necessarily associated with kinematic heating. Additionally, it is an open question whether or not an episode of cold torquing is kinemically cold over long radial distances. This study uses a suite of analytically based simulations to investigate the dynamical response when stars are subject to cold torquing and are also resonant with an ultraharmonic. Model results demonstrate that these populations are kinematically heated and have RMS changes in orbital angular momentum around corotation that can exceed those of populations that do not experience resonant overlap. Thus, kinematic heating can occur during episodes of cold torquing. In a case study of a Milky Way-like disk with an exponential surface density profile and flat rotation curve, up to 40% of cold torqued stars in the solar cylinder experience resonant overlap. This fraction increases toward the galactic center. To first approximation, the maximum radial excursions from cold torquing depend only on the strength of the spiral pattern and the underlying rotation curve. This work places an upper limit to these excursions to be the distance between the ultraharmonics, otherwise radial migration near corotation can kinematically heat. The diffusion rate for kinematically cold radial migration is thus constrained by limiting the step size in the random walk approximation.

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arXiv2019

A profile in FIRE: resolving the radial distributions of satellite galaxies in the Local Group with simulations

While many tensions between Local Group (LG) satellite galaxies and LCDM cosmology have been alleviated through recent cosmological simulations, the spatial distribution of satellites remains an important test of physical models and physical versus numerical disruption in simulations. Using the FIRE-2 cosmological zoom-in baryonic simulations, we examine the radial distributions of satellites with Mstar > 10^5 Msun around 8 isolated Milky Way- (MW) mass host galaxies and 4 hosts in LG-like pairs. We demonstrate that these simulations resolve the survival and physical destruction of satellites with Mstar >~ 10^5 Msun. The simulations broadly agree with LG observations, spanning the radial profiles around the MW and M31. This agreement does not depend strongly on satellite mass, even at distances <~ 100 kpc. Host-to-host variation dominates the scatter in satellite counts within 300 kpc of the hosts, while time variation dominates scatter within 50 kpc. More massive host galaxies within our sample have fewer satellites at small distances, likely because of enhanced tidal destruction of satellites via the baryonic disks of host galaxies. Furthermore, we quantify and provide fits to the tidal depletion of subhalos in baryonic relative to dark matter-only simulations as a function of distance. Our simulated profiles imply observational incompleteness in the LG even at Mstar >~ 10^5 Msun: we predict 2-10 such satellites to be discovered around the MW and possibly 6-9 around M31. To provide cosmological context, we compare our results with the radial profiles of satellites around MW analogs in the SAGA survey, finding that our simulations are broadly consistent with most SAGA systems.

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arXiv2018

Synthetic Gaia surveys from the FIRE cosmological simulations of Milky Way-mass galaxies

With Gaia Data Release 2, the astronomical community is entering a new era of multidimensional surveys of the Milky Way. This new phase-space view of our Galaxy demands new tools for comparing observations to simulations of Milky-Way-mass galaxies in a cosmological context, to test the physics of both dark matter and galaxy formation. We present ananke, a framework for generating synthetic phase-space surveys from high-resolution baryonic simulations, and use it to generate a suite of synthetic surveys resembling Gaia DR2 in data structure, magnitude limits, and observational errors. We use three cosmological simulations of Milky-Way-mass galaxies from the Latte suite of the Feedback In Realistic Environments (FIRE) project, which feature self-consistent clustering of star formation in dense molecular clouds and thin stellar/gaseous disks in live cosmological halos with satellite dwarf galaxies and stellar halos. We select three solar viewpoints from each simulation to generate nine synthetic Gaia-like surveys. We sample synthetic stars by assuming each star particle (of mass 7070 $M_{\odot}$) represents a single stellar population. At each viewpoint, we compute dust extinction from the simulated gas metallicity distribution and apply a simple error model to produce a synthetic Gaia-like survey that includes both observational properties and a pointer to the generating star particle. We provide the complete simulation snapshot at $z = 0$ for each simulated galaxy. We describe data access points, the data model, and plans for future upgrades. These synthetic surveys provide a tool for the scientific community to test analysis methods and interpret Gaia data.

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arXiv2016

Detailed Chemical Abundances in the r-Process-Rich Ultra-Faint Dwarf Galaxy Reticulum 2

The ultra-faint dwarf 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 CS22892-052. Such r-process-enhanced stars have not been found in any other ultra-faint dwarf 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 alpha-enhancement (mean [Mg/Fe]=+0.34+/-0.03). The abundances of other alpha and Fe-group elements closely resemble those in ultra-faint dwarf 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.

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