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Abstract Photometric stellar surveys now cover a large fraction of the sky, probe to fainter magnitudes than large-scale spectroscopic surveys, and are relatively free from the target selection biases often associated with such studies. Photometric-metallicity estimates that include narrow/medium-band filters can achieve comparable accuracy and precision to existing low-resolution spectroscopic surveys such as Sloan Digital Sky Survey/SEGUE and LAMOST. Here we report on an effort to identify likely members of the Galactic disk system among the very metal-poor (VMP; [Fe/H] ≤ −2) and extremely metal-poor (EMP; [Fe/H] ≤ −3) stars. Our analysis is based on an initial sample of ∼11.5 million stars with full space motions selected from the SkyMapper Southern Survey (SMSS) and Stellar Abundance and Galactic Evolution Survey (SAGES). After applying a number of quality cuts to obtain the best available metallicity and dynamical estimates, we analyze a total of ∼5.86 million stars in the combined SMSS/SAGES sample. We employ two techniques that, depending on the method, identify between 876 and 1476 VMP stars (6.9%−11.7% of all VMP stars) and between 40 and 59 EMP stars (12.4%−18.3% of all EMP stars) that appear to be members of the Galactic disk system on highly prograde orbits (v_ϕ> 150 km s⁻¹). The total number of candidate VMP/EMP disklike stars is 1496, the majority of which have low orbital eccentricities, ecc ≤ 0.4; many have ecc ≤ 0.2. The large fractions of VMP/EMP stars associated with the Milky Way disk system strongly suggest the presence of an early-forming “primordial” disk. 

ABSTRACT We employ a sample of 135 873 RR Lyrae stars (RRLs) with precise photometric-metallicity and distance estimates from the newly calibrated P–ϕ31–R21–[Fe/H] and Gaia G band P–R21–[Fe/H] absolute magnitude–metallicity relations of Li et al., combined with available proper motions from Gaia EDR3, and 6955 systemic radial velocities from Gaia DR3 and other sources, in order to explore the chemistry and kinematics of the halo of the Milky Way (MW). This sample is ideally suited for characterization of the inner- and outer-halo populations of the stellar halo, free from the bias associated with spectroscopically selected probes, and for estimation of their relative contributions as a function of Galactocentric distance. The results of a Gaussian mixture model analysis of these contributions are broadly consistent with other observational studies of the halo, and with expectations from recent MW simulation studies. We apply the hdbscan clustering method to the specific energies and cylindrical actions (E, Jr, Jϕ, Jz), identifying 97 dynamically tagged groups (DTGs) of RRLs, and explore their associations with recognized substructures of the MW. The precise photometric-distance determinations (relative distance errors on the order of 5 per cent or better), and the resulting high-quality determination of dynamical parameters, yield highly statistically significant (low) dispersions of [Fe/H] for the stellar members of the DTGs compared to random draws from the full sample, indicating that they share common star-formation and chemical histories, influenced by their birth environments. 

Abstract We construct a sample of 644 carbon-enhanced metal-poor (CEMP) stars with abundance analyses based on moderate- to high-resolution spectroscopic studies. Dynamical parameters for these stars are estimated based on radial velocities, Bayesian parallax-based distance estimates, and proper motions from Gaia EDR3 and DR3, supplemented by additional available information where needed. After separating our sample into the different CEMP morphological groups in the Yoon–Beers diagram of absolute carbon abundance versus metallicity, we used the derived specific energies and actions ( E , J r , J ϕ , J z ) to cluster them into Chemodynamically Tagged Groups (CDTGs). We then analyzed the elemental-abundance dispersions within these clusters by comparing them to the dispersion of clusters that were generated at random. We find that, for the Group I (primarily CEMP- s and CEMP- r / s ) clustered stars, there exist statistically insignificant intracluster dispersions in [Fe/H], [C/Fe] c (evolution corrected carbon), and [Mg/Fe] when compared to the intracluster dispersions of randomly clustered Group I CEMP stars. In contrast, the Group II (primarily CEMP-no) stars exhibit clear similarities in their intracluster abundances, with very low, statistically significant, dispersions in [C/Fe] c and marginally significant results in [Mg/Fe]. These results strongly indicate that Group I CEMP stars received their carbon enhancements from local phenomena, such as mass transfer from an evolved binary companion in regions with extended star formation histories, while the CDTGs of Group II CEMP stars formed in low-metallicity environments that had already been enriched in carbon, likely from massive rapidly rotating ultra- and hyper-metal-poor stars and/or supernovae associated with high-mass early-generation stars. 

Abstract Orbital characteristics based on Gaia Early Data Release 3 astrometric parameters are analyzed for ∼1700r-process-enhanced (RPE; [Eu/Fe] > +0.3) metal-poor stars ([Fe/H] ≤ −0.8) compiled from theR-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 theHDBSCANunsupervised learning algorithm. We identify 36 chemodynamically tagged groups (CDTGs) containing between five and 22 members; 17 CDTGs have at least 10 member stars. Previously known Milky Way (MW) substructures such as Gaia-Sausage-Enceladus, the splashed disk, the metal-weak thick disk, the Helmi stream, LMS-1 (Wukong), and Thamnos are reidentified. Associations with MW globular clusters are determined for seven CDTGs; no recognized MW dwarf galaxy satellites were associated with any of our CDTGs. Previously identified dynamical groups are also associated with our CDTGs, adding structural determination information and possible new identifications. Carbon-enhanced metal-poor RPE (CEMP-r) stars are identified among the targets; we assign these to morphological groups in a Yoon–BeersA(C)_cversus [Fe/H] diagram. Our results confirm previous dynamical analyses that showed RPE stars in CDTGs share common chemical histories, influenced by their birth environments. 

ABSTRACT The r-process-enhanced (RPE) stars provide fossil records of the assembly history of the Milky Way (MW) and the nucleosynthesis of the heaviest elements. Observations by the R-Process Alliance (RPA) and others have confirmed that many RPE stars are associated with chemo-dynamically tagged groups, which likely came from accreted dwarf galaxies of the MW. However, we do not know how RPE stars are formed. Here, we present the result of a cosmological zoom-in simulation of an MW-like galaxy with r-process enrichment, performed with the highest resolution in both time and mass. Thanks to this advancement, unlike previous simulations, we find that most highly RPE (r-II; [Eu/Fe] > +0.7) stars are formed in low-mass dwarf galaxies that have been enriched in r-process elements for [Fe/H] $$\lt -2.5$$, while those with higher metallicity are formed in situ, in locally enhanced gas clumps that were not necessarily members of dwarf galaxies. This result suggests that low-mass accreted dwarf galaxies are the main formation site of r-II stars with [Fe/H] $$\, \lt -2.5$$. We also find that most low-metallicity r-II stars exhibit halo-like kinematics. Some r-II stars formed in the same halo show low dispersions in [Fe/H] and somewhat larger dispersions of [Eu/Fe], similar to the observations. The fraction of simulated r-II stars is commensurate with observations from the RPA, and the distribution of the predicted [Eu/Fe] for halo r-II stars matches that observed. These results demonstrate that RPE stars can be valuable probes of the accretion of dwarf galaxies in the early stages of their formation.