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Abstract Massive Population III (Pop III) stars are currently not observed, but their initial mass function (IMF) can be inferred through stellar archaeology: by fitting core-collapse supernova yield models to elemental abundances of low-mass, long-lived metal-poor stars. While prior work demonstrates that yield fitting can recover progenitor properties, it remains unclear which measured elements most control mass recovery quality and what level of IMF precision is achievable for a measured element set. We perform a systematic study of element importance for progenitor mass recovery. Using the A. Heger & S. E. Woosley (2010) yield grid, we generate mock observations, fit the initial mass, and evaluate the typical performance on the fractional mass recovery. Add-/remove-one-element experiments and comparisons among different baseline element sets are used to rank elements by importance. We find that the most important elements for accurate mass recovery are C, N, Na, and K, with O, Al, Co, and Ni consistently improving performance when available. Overall, with currently measurable elements from high-resolution spectroscopy, stellar archaeology can deliver practical Pop III IMF constraints, assuming the core-collapse supernova yield models provide a good representation of stellar evolution in the early Universe. 

Abstract Extremely metal-poor stars are intrinsically rare, but emerging methods exist to accurately classify them from all-sky Gaia XP low-resolution spectra. To assess their overall accuracy for targeting metal-poor stars, we present a high-resolution spectroscopic followup of 75 very metal-poor candidates selected from the catalog by R. Andrae, V. Chandra, and H. W. Rix. We discover two new extremely metal-poor ([Fe/H] < −3) stars and 20 new very metal-poor ([Fe/H] < −2) stars. Abundances of up to 22 elements are derived from 1D local thermodynamic equilibrium analysis, and kinematic parameters are derived using Gaia astrometry and spectroscopic radial velocities. The chemodynamical properties are mostly consistent with expectations for halo stars, but we discover a Mg-enhanced CEMP star ([Mg/Fe] = 0.89) and a Mg-poor star from an accreted ultrafaint dwarf galaxy. The Gaia XP metallicity estimates are consistent with our [Fe/H] measurements down to [Fe/H] ∼ −3.0, but estimates worsen in highly extincted regions. We find that four other XP-based metallicity catalogs succeed in mitigating contaminants and can also classify metal-poor stars robustly to [Fe/H] ∼ −3.0. Our results demonstrate the utility of Gaia XP spectra for identifying the most metal-poor stars across the Galaxy. 

Abstract Stellar surface abundances are records of the state of the gas from which stars formed and thus trace how individual elements have mixed into the surrounding medium following their ejection from stars. In this work, we test the common assumption of instantaneous and homogeneous metal mixing during the formation of the first Population II stars by characterizing the chemical homogeneity of the gas in simulated star-forming environments enriched by Population III stellar feedback. Testing the homogeneity of metal mixing in this time period is necessary for understanding the spread of abundances in the most metal-poor stars and the (in)homogeneity of individual sites of star formation. Using Aeos, a suite of star-by-star cosmological simulations, we quantify how gas abundances change over space and time relative to Population II stellar abundances using Mahalanobis distances, a measure of covariance-normalized dissimilarity. We find that the homogeneous mixing assumption holds only within ∼100 pc of a star-forming region and ∼7 Myr following the star formation event. Beyond this regime, deviations between stellar and gas abundances increase until they become indistinguishable from assuming a homogeneous mix of metals averaged over the initial mass function. This highlights the limited applicability of assuming instantaneous and homogeneous mixing in realistic halo environments at high redshift. We identify critical mixing scales that are necessary to explore chemical evolution in the early Universe. These scales can be applied to determine the precision needed for accurate chemical tagging of observed data and to explore parameter space with analytical models. 

Abstract A substantial fraction of metal-poor stars in the local Milky Way halo exhibit large overabundances of carbon. These stars, dubbed carbon-enhanced metal-poor (CEMP) stars, provide crucial constraints on the nature of the early Universe, including the earliest nucleosynthetic events. Whether these stars exist at similar rates in nearby galaxies is a major open question with implications for the environmental dependence of early chemical evolution. Here, we present the discovery of the first five CEMP stars in the Milky Way’s largest dwarf companion, the LMC, using fifth-generation Sloan Digital Sky Survey (SDSS-V) spectra from the Baryon Oscillation Spectroscopic Survey instrument. We measure metallicities ranging from [Fe/H] = −2.1 to −3.2 and evolutionary state–corrected carbon enhancements of [C/Fe] = +1.2 to +2.4, placing these stars among the most metal-poor and carbon-rich ever identified in the LMC. Their absolute carbon abundances and metallicities classify them as Group I CEMP stars, suggesting binary mass-transfer origins, though neutron-capture abundance measurements are required to confirm whether this classification scheme applies beyond the Milky Way. Although these stars were selected as the most promising CEMP candidates from the SDSS-V sample, likely biasing this initial sample toward higher absolute carbon abundances, their discovery suggests that previous null detections of CEMP stars in the LMC were caused by metallicity-sensitive photometric targeting biases against high [C/H] stars. A forthcoming analysis of the full spectroscopic sample will push to lower carbon abundances, providing a more complete census and enabling critical tests of whether environmental differences shape the formation channels and frequencies of CEMP stars in this system. 

The chemical abundances of stars in galaxies are a fossil record of the star formation and stellar evolution processes that regulate galaxy formation, including the stellar initial mass function, the fraction and timing of type Ia supernovae (SNeIa), and nucleosynthesis inside massive stars. In this paper, we systematically explore uncertainties associated with modeling chemical enrichment in dwarf galaxies. We repeatedly simulate a singleEDGE-INFERNOdwarf (M_★ ≈ 10⁵ M_⊙), varying the chemical yields of massive stars, the timing and yields of SNeIa, and the intrinsic stochasticity that arises from sampling individual stars and galaxy formation chaoticity. All simulations are high-resolution (3.6 pc), cosmological zoom-in hydrodynamical simulations that track the stellar evolution of all individual stars with masses of > 0.5 M_⊙. We find that SNeIa make significant contributions to the iron content of low-mass, reionization-limited galaxies, with possible variations in mean abundance ratios and [Fe/H] related to minor changes in their evolutionary timescales. In contrast, different massive star yields, accounting (or not) for stellar rotation, result in mean abundance variations comparable to those arising from stochasticity, with the possible exception of extremely rapidly rotating stars. Nonetheless, massive stars significantly affect the shape of abundance trends with [Fe/H], for example, through the existence (or not) of a bimodality in the [X/Fe]–[Fe/H] planes, particularly in [Al/Fe]. Finally, we find that the variance arising from random sampling severely limits the interpretation of single galaxies. Our analysis showcases the power of star-by-star cosmological models to unpick how both systematic uncertainties (e.g., assumptions in low-metallicity chemical enrichment) and statistical uncertainties (e.g., averaging over enough galaxies and stars within a galaxy) affect the interpretation of chemical observables in ultra-faint dwarf galaxies. 

Abstract Owing to their proximity to the Milky Way, the Large and Small Magellanic Clouds (L/SMC) uniquely probe the evolution of low-mass galaxies undergoing mutual interactions. In this work, we investigate the connection between the star formation histories (SFHs) of the L/SMC measured from Hubble Space Telescope imaging in the Scylla survey and APOGEE chemical abundances. We model the chemical evolution of the L/SMC in the [Mg/Fe]–[Fe/H] plane within a robust statistical framework to predict chemical abundance signatures resulting directly from starbursts in Scylla SFHs. Both the L/SMC rapidly enrich to high metallicity ([Fe/H] ≳ −1) within 3 Gyr, followed by slower chemical evolution regulated by sequential starbursts, where the SMC may require higher Fe yields from Type Ia supernovae than the LMC. We also model the [Mg/Fe]–[Fe/H] plane to infer starburst properties across distinct spatial regions in the L/SMC. We identify dominant starbursts in the L/SMC with broadly similar timing, though the SMC may host an earlier burst and larger burst strength in the LMC. The global starburst properties are nearly uniform across the LMC disk, whereas the dominant SMC population experiences a stronger and later-onset burst in its eastern wing compared to the main body. We also find evidence for a chemically distinct secondary population in the SMC that preferentially traces the foreground and may originate from the LMC. We discuss the implications of these results for the evolutionary history of the L/SMC and for starbursts in interacting low-mass galaxy pairs. 

Abstract Neutron star (NS) mergers are currently the only observed source ofr-process production in the Universe. Yet, it is unclear how muchr-process mass from these mergers is incorporated into star-forming gas to enrich stars. This is crucial to consider as all otherr-process mass estimates in the Universe beyond Earth are based on stellarr-process abundances. Here, we explore the extent to which merger location and host-galaxy properties affect the incorporation ofr-process elements into star-forming gas, and quantify an “enrichment” timescale to account for this process. To put this timescale in context, we analyze a population of 12 gamma-ray bursts (GRBs) with probable associations tor-process kilonovae (GRB-KNe) and 74 short GRBs without claimed KNe, including new nonparametric star formation histories for the GRB-KN hosts. We find the enrichment timescales for this sample are between ​​​​​​≈7 Myr and 1.6 Gyr, suggesting that environmental enrichment is delayed from NS merger occurrence. Moreover, we find a correlation between the amount of environmental enrichment from a single event and increasing host specific star formation rate (sSFR), and little correlation with stellar mass and GRB galactocentric offset. Environments with low sSFRs (<10^−10.5yr⁻¹), which comprise 18% of short-GRB hosts and the host of GW170817, will have little to no capacity for stellar enrichment. Our results indicate that not allr-process from NS mergers is incorporated into newly forming stars, and instead some remains “lost” to the circumgalactic medium or intergalactic medium. Future studies should consider these losses to understand the total contribution from NS mergers to the Universe’sr-process budget. 

Abstract The extremely low-luminosity, compact Milky Way satellite Ursa Major III/UNIONS 1 (UMaIII/U1;L_V = 11L_⊙,a_1/2 = 3 pc) was found to have a substantial velocity dispersion at the time of its discovery ( ${\sigma }_v=3.7_{-1.0}^{+1.4}\mathrm{km}{\mathrm{s}}^{-1}$), suggesting that it might be an exceptional, highly dark-matter-dominated dwarf galaxy with very few stars. However, significant questions remained about the system’s dark matter content and nature as a dwarf galaxy, due to the small member sample (N= 11), possible spectroscopic binaries, and the lack of any metallicity information. Here, we present new spectroscopic observations coveringN= 16 members that both dynamically and chemically test the true nature of UMaIII/U1. From higher-precision Keck/DEIMOS spectra, we find a 95% confidence level velocity dispersion limit ofσ_v < 2.3 km s⁻¹, with a ∼120:1 likelihood ratio favoring the expected stellar-only dispersion ofσ_* ≈ 0.1 km s⁻¹over the original 3.7 km s⁻¹dispersion. There is now no observational evidence for dark matter in the system. From Keck/LRIS spectra targeting the CaIIK line, we also measure the first metallicities for 12 member stars, finding a mean metallicity of [Fe/H] = − 2.65 ±  0.1 (stat.) ±0.3 (zero-point), with a metallicity dispersion limit ofσ_[Fe/H] < 0.35 dex (at the 95% credible level). Together, these properties are more consistent with UMaIII/U1 being a star cluster, though the dwarf galaxy scenario is not fully ruled out. Under this interpretation, UMaIII/U1 ranks among the faintest and most metal-poor star clusters yet discovered.