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1. Beyond Spectroscopy. II. Stellar Parameters for over 20 Million Stars in the Northern Sky from SAGES DR1 and Gaia DR3

   https://doi.org/10.3847/1538-4357/ace628  

   Huang, Yang; Beers, Timothy C; Yuan, Haibo; Tan, Ke-Feng; Wang, Wei; Zheng, Jie; Li, Chun; Lee, Young Sun; Li, Hai-Ning; Zhao, Jing-Kun; et al (October 2023, The Astrophysical Journal)

   Abstract We present precise photometric estimates of stellar parameters, including effective temperature, metallicity, luminosity classification, distance, and stellar age, for nearly 26 million stars using the methodology developed in the first paper of this series, based on the stellar colors from the Stellar Abundances and Galactic Evolution Survey (SAGES) Data Release 1 and Gaia Early Data Release 3. The optimal design of stellar-parameter sensitiveuvfilters by SAGES has enabled us to determine photometric-metallicity estimates down to −3.5, similar to our previous results with the SkyMapper Southern Survey (SMSS), yielding a large sample of over five million metal-poor ([Fe/H] ≤ −1.0) stars and nearly one million very metal-poor ([Fe/H] ≤ −2.0) stars. The typical precision is around 0.1 dex for both dwarf and giant stars with [Fe/H] > −1.0, and 0.15–0.25/0.3–0.4 dex for dwarf/giant stars with [Fe/H] < −1.0. Using the precise parallax measurements and stellar colors from Gaia, effective temperature, luminosity classification, distance, and stellar age are further derived for our sample stars. This huge data set in the Northern sky from SAGES, together with similar data in the Southern sky from SMSS, will greatly advance our understanding of the Milky Way, in particular its formation and evolution. 

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2. Candidate Members of the VMP/EMP Disk System of the Galaxy from the SkyMapper and SAGES Surveys

   https://doi.org/10.3847/1538-4365/ad4a6f  

   Hong, Jihye; Beers, Timothy C; Lee, Young Sun; Huang, Yang; Hirai, Yutaka; Cabrera_Garcia, Jonathan; Shank, Derek; Xu, Shuai; Yuan, Haibo; Mardini, Mohammad K; et al (July 2024, The Astrophysical Journal Supplement Series)

   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. 

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3. Metallicities of 20 Million Giant Stars Based on Gaia XP Spectra

   https://doi.org/10.3847/1538-4365/add5e3  

   Yang_杨, Lin 琳; Yuan_苑, Haibo 海波; Huang_黄, Bowen 博闻; Zhang_张, Ruoyi 若羿; Beers, Timothy C; Xiao_肖, Kai 凯; Xu_徐, Shuai 帅; Huang_黄, Yang 样; Xiang_向, Maosheng 茂盛; Zhang_张, Meng 萌; et al (June 2025, The Astrophysical Journal Supplement Series)

   Abstract We design an uncertainty-aware cost-sensitive neural network (UA-CSNet) to estimate metallicities from dereddened and corrected Gaia BP/RP (XP) spectra for giant stars. This method accounts for both stochastic errors in the input spectra and the imbalanced density distribution in [Fe/H] values. With a specialized architecture and training strategy, the UA-CSNet improves the precision of the predicted metallicities, especially for very metal-poor (VMP; [Fe/H] ≤ −2.0) stars. With the PASTEL catalog as the training sample, our model can estimate metallicities down to [Fe/H] ∼ −4. We compare our estimates with a number of external catalogs and conduct tests using star clusters, finding overall good agreement. We also confirm that our estimates for VMP stars are unaffected by carbon enhancement. Applying the UA-CSNet, we obtain reliable and precise metallicity estimates for approximately 20 million giant stars, including 360,000 VMP stars and 50,000 extremely metal-poor ([Fe/H] ≤ −3.0) stars. The resulting catalog is publicly available via the Chinese Virtual Observatory at doi: 10.12149/101604. This work highlights the potential of low-resolution spectra for metallicity estimation and provides a valuable data set for studying the formation and chemodynamical evolution of our Galaxy. 

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4. J-PLUS: Beyond Spectroscopy. III. Stellar Parameters and Elemental-abundance Ratios for Five Million Stars from DR3

   https://doi.org/10.3847/1538-4357/ad6b94  

   Huang_黄, Yang 样; Beers, Timothy C; Xiao_肖, Kai 凯; Yuan_苑, Haibo 海波; Lee, Young Sun; Gu_顾, Hongrui 弘睿; Hong, Jihye; Liu_刘, Jifeng 继峰; Fan_范, Zhou 舟; Coelho, Paula; et al (October 2024, The Astrophysical Journal)

   Abstract We present a catalog of stellar parameters (effective temperatureT_eff, surface gravity $\log g$, age, and metallicity [Fe/H]) and elemental-abundance ratios ([C/Fe], [Mg/Fe], and [α/Fe]) for some five million stars (4.5 million dwarfs and 0.5 million giant stars) in the Milky Way, based on stellar colors from the Javalambre Photometric Local Universe Survey (J-PLUS) DR3 and Gaia EDR3. These estimates are obtained through the construction of a large spectroscopic training set with parameters and abundances adjusted to uniform scales, and trained with a kernel principal component analysis. Owing to the seven narrow/medium-band filters employed by J-PLUS, we obtain precisions in the abundance estimates that are as good as or better than those derived from medium-resolution spectroscopy for stars covering a wide range of the parameter space: 0.10–0.20 dex for [Fe/H] and [C/Fe], and 0.05 dex for [Mg/Fe] and [α/Fe]. Moreover, systematic errors due to the influence of molecular carbon bands on previous photometric-metallicity estimates (which only included two narrow/medium-band blue filters) have now been removed, resulting in photometric-metallicity estimates down to [Fe/H] ∼ −4.0, with typical uncertainties of 0.40 dex and 0.25 dex for dwarfs and giants, respectively. This large photometric sample should prove useful for the exploration of the assembly and chemical-evolution history of our Galaxy. 

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5. Metallicity Distribution Functions of 13 Ultra-faint Dwarf Galaxy Candidates from Hubble Space Telescope Narrowband Imaging

   https://doi.org/10.3847/1538-4357/ad0030  

   Fu, Sal Wanying; Weisz, Daniel R; Starkenburg, Else; Martin, Nicolas; Savino, Alessandro; Boylan-Kolchin, Michael; Côté, Patrick; Dolphin, Andrew E; Ji, Alexander P; Longeard, Nicolas; et al (November 2023, The Astrophysical Journal)

   Abstract We present uniformly measured stellar metallicities of 463 stars in 13 Milky Way (MW) ultra-faint dwarf galaxies (UFDs;M_V= −7.1 to −0.8) using narrowband CaHK (F395N) imaging taken with the Hubble Space Telescope. This represents the largest homogeneous set of stellar metallicities in UFDs, increasing the number of metallicities in these 13 galaxies by a factor of 5 and doubling the number of metallicities in all known MW UFDs. We provide the first well-populated MDFs for all galaxies in this sample, with 〈[Fe/H]〉 ranging from −3.0 to −2.0 dex, andσ_[Fe/H]ranging from 0.3–0.7 dex. We find a nearly constant [Fe/H]∼ −2.6 over 3 decades in luminosity (∼10²–10⁵L_⊙), suggesting that the mass–metallicity relationship does not hold for such faint systems. We find a larger fraction (24%) of extremely metal-poor ([Fe/H]< −3) stars across our sample compared to the literature (14%), but note that uncertainties in our most metal-poor measurements make this an upper limit. We find 19% of stars in our UFD sample to be metal-rich ([Fe/H] > −2), consistent with the sum of literature spectroscopic studies. MW UFDs are known to be predominantly >13 Gyr old, meaning that all stars in our sample are truly ancient, unlike metal-poor stars in the MW, which have a range of possible ages. Our UFD metallicities are not well matched to known streams in the MW, providing further evidence that known MW substructures are not related to UFDs. We include a catalog of our stars to encourage community follow-up studies, including priority targets for ELT-era observations. 

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