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Abstract We present a detailed chemical-abundance and kinematic analysis of four extremely metal-poor (EMP; [Fe/H] ≤ −3.0) stars identified from Gaia BP–RP data in our ongoing search for the most primitive stars. This includes a primary target, Gaia DR3 2563539603865382656 (hereafter G256353), a stronglyr-process-enhanced star with [Eu/Fe] = +1.20 and [Ba/Eu] = −0.64. Our results are based on high-resolution, high-signal-to-noise-ratio GHOST spectra from Gemini-South. For the full sample, we statistically match the light-element abundances with those predicted from Population III supernova models. The best-fit model suggests massive progenitors with stellar masses ofM_⋆ ∼ 20–30M_⊙. In addition, we determine orbital histories for all of the stars. We find that Gaia DR3 2887334237669844480 appears to be kinematically associated with Atari, an accreted structure in the Galactic disk. This star has low abundance ratios of strontium ([Sr/Fe] = −1.09) and barium ([Ba/Fe] = −0.37), which support an accretion origin. For G256353, we determine chemical abundances for 15 neutron-capture elements. We compare the observed heavy-element pattern for G256353 with that of the Sun, HD 222925, and two neutron star merger models. Ther-process elements in G256353 align reasonably well with HD 222925, the scaled-solar pattern (except for the first peak), and a recent predicted pattern associated with neutron star mergers. This consistency reinforces the universality of the mainr-process across diverse astrophysical environments. 

Abstract As part of the ongoing work of theR-Process Alliance (RPA), detailed abundance measurements of 29 heavy elements in three metal-poor stars, 2MASS J14592981−3852558, 2MASS J19445483−4039459, and 2MASS J15211026−0607566, are presented based on an analysis of high-resolution (R ∼ 80,000), high-signal-to-noise “portrait” spectra from the Magellan Inamori Kyocera Echelle spectrograph on the Magellan Clay Telescope at Las Campanas Observatory. The selected targets were identified asr-process-enhanced metal-poor stars in previous RPA snapshot analyses. They have also been linked to possible chemodynamically tagged groups, indicating that the stars may have formed in dwarf galaxies that were later accreted into the Milky Way halo. These stars have also been tentatively linked to the Thamnos structure. The detailed chemical abundances in this work confirm that 2MASS J14592981−3852558 and J15211026−0607566 arer-II stars, while 2MASS J19445483−4039459 is found to lie just below the threshold forr-I status. Ther-II stars show signs of slight enhancement in fission fragments compared to 2MASS J19445483−4039459. Based on radioactive age dating with Th, ther-process material in the twor-II stars is found to be old (with ages > 10 Gyr); neither star shows signs of an actinide boost. The varying elemental compositions suggest that these stars likely did not originate in the same environment, though each could be consistent with originating in the Thamnos progenitor. 

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 Recent measurements of the Milky Way rotation curve found a sharp decline at around 15–20 kpc from the center of the Galaxy, suggesting that the Galactic dark matter halo is much less massive than predicted by other dynamical tracers. To address this tension, we study the validity of the assumptions made in calculating the Milky Way’s rotation curve. To do so, we apply the Jeans equation, the current standard approach of measuring rotation curves, to three cosmological zoom-in simulations of Milky Way–like galaxies from theFIRE-2 Latte suite. Using synthetic Gaia surveys, we replicate the sample selection process and calculation employed in measuring the Milky Way rotation curve. We examine four failure modes of this calculation and find that the measured curves deviate from the true curve by 5%–20% rather than below 5%, as estimated by previous works. Interestingly, there is a large galaxy-to-galaxy variance, and different systematics dominate different galaxies. We rederive the Milky Way’s dark matter density profile with the rotation curve while incorporating systematics from the simulations. The posterior distribution of the density profiles is consistent with a fiducial Navarro–Frenk–White (NFW) profile when assuming a generalized NFW profile for dark matter. We find that the virial mass is $7.32_{-1.53}^{+1.98}\times 10^{11}$ M_⊙, consistent with other probes of the Milky Way’s mass. However, we recommend that the field move away from relying solely on the rotation curve when studying the dark matter profile and adopt methods that incorporate additional probes and/or do not heavily depend on assumptions described in this study. 

Abstract 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 anN-body/smoothed particle hydrodynamics code,asura. Most stars with no overabundance inr-process elements, as well as the stronglyr-process-enhanced (RPE)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 enhancedr-I stars (+0.3 < [Eu/Fe] ≤ +0.7) are formed in the main in situ disk after 6 Gyr. Our results suggest that the fraction ofr-I andr-II stars formed in disrupted dwarf galaxies is larger the higher their [Eu/Fe] is. Accordingly, the most strongly enhancedr-III stars ([Eu/Fe] > +2.0) are formed in accreted components. These results suggest that non-r-process-enhanced stars andr-II stars are mainly formed in low-mass dwarf galaxies that hosted either none or a single neutron star merger, while ther-I stars tend to form in the well-mixed in situ disk. We compare our findings with high-resolution spectroscopic observations of RPE metal-poor stars in the halo and dwarf galaxies, including those collected by theR-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. 

Abstract We explore the effect of variations in the Population III initial mass function (IMF) and star-by-star feedback on early galaxy formation and evolution using the Aeossimulations. We compare simulations with two different Population III IMFs:M_char = 10M_⊙and $M_{\max }=100M_{\odot }$ (Aeos10) andM_char = 20M_⊙and $M_{\max }=300M_{\odot }$ (Aeos20). Aeos20 produces significantly more ionizing photons, ionizing 30% of the simulation volume byz ≈ 14, compared to 9% in Aeos10. This enhanced ionization suppresses galaxy formation on the smallest scales. Differences in Population III IMF also affect chemical enrichment. Aeos20 produces Population II stars with higher abundances, relative to iron, of light andα-elements, a stronger odd–even effect, and a higher frequency of carbon-enhanced metal-poor stars. The abundance scatter between different Population II galaxies dominates the differences due to Population III IMF, though, implying a need for a larger sample of Population II stars to interpret the impact of Population III IMF on early chemical evolution. We also compare the Aeossimulations to traditional simulations that use single stellar population particles. We find that star-by-star modeling produces a steeper mass–metallicity relation due to less bursty feedback. These results highlight the strong influence of the Population III IMF on early galaxy formation and chemical evolution, emphasizing the need to account for IMF uncertainties in simulations and the importance of metal-poor Population II stellar chemical abundances when studying the first stars. 

Abstract We report on the discovery of GDR3_526285 (Gaia DR3 Source ID 5262850721755411072), a star with [Fe/H] = −4.82 ± 0.25 and one of the lowest metal (atomic number > 2) mass fractions ever found (Z_GDR3_₅₂₆₅₈₅ ≲ 1.0 × 10⁻⁶). We first identified it as an ultra-metal-poor (UMP; [Fe/H] < −4) red-giant branch (RGB) star candidate in the Gaia Blue (BP) and Red (RP) Photometer (XP) spectrophotometric catalog (GaiaGmagnitude ≈ 15). A combination of multiband photometry and high-resolution spectroscopic analysis under local thermodynamic equilibrium confirmed the status of GDR3_526285 as a distant (≈24 kpc from the Sun) RGB star (T_eff = 4596 K, $\log g=0.88$ ) in the Milky Way’s outer halo. We obtain only an upper limit for the carbon abundance of [C/H] < −4.32, resulting in [C/Fe] < +0.50. A correction for the evolutionary carbon depletion (Δ[C/Fe] = +0.68) brings the nominal carbon-to-iron ratio upper limit to [C/Fe]_cor < +1.18. Given its extraordinarily low [C/H], GDR3_526285 likely formed from gas cooled via dust grains rather than fine structure line cooling. The kinematics of GDR3_526285 suggests that this star was either dynamically perturbed by the infall of the Magellanic system or was formerly a member of the Magellanic Clouds and was later stripped by the Milky Way. Our results showcase the potential of an all-sky search for low-metallicity targets with Gaia XP and confirm that the methodology described here is a useful “treasure map” for finding additional UMP stars. 

Abstract Low-metallicity stars preserve the signatures of the first stellar nucleosynthesis events in the Galaxy, as their surface abundances reflect the composition of the interstellar medium from the time when they were born. Aside from primordial Big Bang nucleosynthesis, massive stars, due to their short lifetimes, dominate the wind and explosive ejecta into the interstellar medium of the early Galaxy. Most of them will end as core-collapse supernova (CCSN) explosions, and typical ejected abundance distributions, e.g. in terms of the$$\alpha $$ $\alpha$ -element-to-Fe ratios, reflect these contributions. Essentially all CCSNe contribute⁵⁶Fe (decaying from radioactive⁵⁶Ni). Therefore, low-metallicity stars can be used to test whether the abundances of any other elements are correlated with those of Fe, i.e. whether these elements have been co-produced in the progenitor sources or if they require either a different or additional astrophysical origin(s). The present analysis focuses on stars with [Fe/H]<-2, as they probe the earliest formation phase of the Galaxy when only one or very few nucleosynthesis events had contributed their ejecta to the gas from which the lowest metallicity stars form. This was also the era before low and intermediate mass stars (or type Ia supernovae) could contribute any additional heavy elements. Following earlier work on the origin of heavy r-process elements [1], we extend the present study to examine Pearson and Spearman correlations of Fe with Li, Be, C, N, O, Na, Mg, Si, S, K, Ca, Ti, Cr, Ni, Zn, Ge, Se, Sr, Y, Zr, Mo, Ba, La, Ce, Sm, Eu, Gd, Dy, Yb, Lu, Hf, Os, Ir, Pb, and Th, using high-resolution stellar abundance data from the SAGA [2] and JINA [3] databases. The main goal is to identify which of the observed elements (i) may have been co-produced with Fe in (possibly a variety of) CCSNe, and which elements require (ii) either a completely different, or (iii) at least an additional astrophysical origin. 

Abstract Studying the abundances in metal-poor globular clusters is crucial for understanding the formation of the Galaxy and the nucleosynthesis processes in the early Universe. We observed 13 red-giant stars from the metal-poor globular cluster NGC 2298 using the newly commissioned GHOST spectrograph at Gemini South. We derived stellar parameters and abundances for 36 species across 32 elements, including 16 neutron-capture elements. We find that the stars exhibit chemical anomalies among the light elements, allowing us to classify them into first generation (eight stars) and second generation (five stars). We derive a mean cluster metallicity of [Fe/H] = −1.98 ± 0.10 with no significant variation among cluster members. Mostα- and Fe-peak elements display low star-to-star abundance dispersion, with notable exceptions for Sc, Ni, and Zn for which the dispersions in Sc vary significantly between stars from different generations to 2σlevels. Similarly, among the neutron-capture elements, we observed considerable differences in dispersion for Sr and Eu among the first and second generation stars to 2σlevels. We also confirm an intrinsic scatter beyond observational uncertainties for several elements using a maximum likelihood approach among stars from different generations. Additionally, we note an increase in [Sr/Eu] and [Ba/Eu] with [Mg/Fe] in first-generation stars indicating correlations between the productions of lightrprocess and Mg. We find the universalr-process pattern, but with larger dispersions in the mainrprocess than the limited-relements. These differences in abundance dispersion, among first- and second-generation stars in NGC 2298, suggest complex and inhomogeneous early chemical enrichment processes, driven by contributions from multiple nucleosynthetic events, including massive stars and rarer-process events. 

Abstract Type Ia supernovae (SNe Ia), critical for studying cosmic expansion, arise from thermonuclear explosions of white dwarfs, but their precise progenitor pathways remain unclear. Growing evidence supports the “double-degenerate scenario,” where two white dwarfs interact. The absence of nondegenerate companions capable of explaining the observed SN Ia rate, along with observations of hypervelocity white dwarfs, interpreted as surviving companions of such systems, provide compelling evidence for this scenario. Upcoming millihertz gravitational-wave observatories like the Laser Interferometer Space Antenna (LISA) are expected to detect thousands of double-degenerate systems, though the most compact known candidate SN Ia progenitors produce marginally detectable signals. Here, we report observations of ATLAS J1138-5139, a binary white dwarf system with an orbital period of just 28 minutes. Our analysis reveals a 1M_☉carbon–oxygen white dwarf accreting from a high-entropy helium-core white dwarf. Given its mass, the accreting carbon–oxygen white dwarf is poised to trigger a typical-luminosity SN Ia within a few million years, to evolve into a stably transferring AM Canum Venaticorum (or AM CVn) system, or undergo a merger into a massive white dwarf. ATLAS J1138-5139 provides a rare opportunity to calibrate binary evolution models by directly comparing observed orbital parameters and mass-transfer rates closer to merger than any known SN Ia progenitor. Its compact orbit ensures detectability by LISA, demonstrating the potential of millihertz gravitational-wave observatories to reveal a population of SN Ia progenitors on a Galactic scale, paving the way for multimessenger studies offering insights into the origins of these cosmologically significant explosions.