Evaluating Classifications of Extremely Metal-poor Candidates Selected from Gaia XP Spectra

TL;DR

This study validates Gaia XP spectra as a practical tool for identifying the Galaxy's oldest stars by performing high-resolution follow-up on 75 XP-selected metal-poor candidates, detecting 2 EMP and 20 VMP stars and deriving abundances for up to 22 elements. The authors demonstrate that Gaia XP-based metallicities reproduce spectroscopic $[Fe/H]$ down to roughly $-3.0$, though extinction can bias estimates, and they compare XP labels with multiple catalogs to assess robustness. The chemodynamical analysis reveals halo-like abundances and orbits, including a Mg-enhanced CEMP star and a Mg-poor star likely originating from an accreted ultra-faint dwarf, underscoring the diversity of early Galactic environments. Overall, the work confirms the utility of Gaia XP spectra for efficiently targeting the most metal-poor stars across the Milky Way and informs refinement of XP-based metallicity catalogs for future surveys.

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 2 new extremely metal-poor ($\rm{[Fe / H]}<-3$) stars and 20 new very metal-poor ($\rm{[Fe/H]} < -2$) 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 an Mg-enhanced CEMP star ($\mathrm{[Mg/Fe]} = 0.89$) and an Mg-poor star from an accreted ultra-faint dwarf galaxy. The Gaia XP metallicity estimates are consistent with our $\rm{[Fe/H]}$ measurements down to $\rm{[Fe/H]}\sim -3.0$, but estimates worsen in highly extincted regions. We find that 4 other XP-based metallicity catalogs succeed in mitigating contaminants and can also classify metal-poor stars robustly to $\rm{[Fe/H]}\sim -3.0$. Our results demonstrate the utility of Gaia XP spectra for identifying the most metal-poor stars across the Galaxy.

Figures (4)

• Figure 1: Stellar parameters for all analyzed compared to scaling relations. Top: effective temperature $T_\mathrm{eff}$ versus surface gravity $\log g$ compared to MIST isochrones choiMesaIsochronesStellar2016 of three different metallicities ($[\mathrm{Fe/H}] = -2.0, -2.5, -3.0$). We generally recover slightly higher or lower $T_\mathrm{eff}$ and $\log(g)$ values than expected from the isochrones. Bottom: surface gravity $\log g$ vs microturbulence $\nu_{t}$ compared to the empirical scaling relations given in barklemHamburgESORprocess2005, marinoSpectroscopicPhotometricEvidence2008, and kirbyMultielementAbundanceMeasurements2009.
• Figure 2: Orbital and kinematic properties for our stars. Left: total energy versus angular momentum, colored by apastron distance $R_{\mathrm{apo}}$. Right: action space, $(J_{z} - J_{r}) / J_{\mathrm{tot}}$ against angular momentum fraction $J_{\phi} / J_{\mathrm{tot}}$. Our orbital properties are consistent with expectations for halo VMP stars. CEMP stars, $r$-process enhanced stars, and magnesium unusual stars are highlighted per the legend.
• Figure 3: Chemical abundances and upper limits $\mathrm{[X / Fe]}$ against metallicity $[\mathrm{Fe/H}]$ for our 21 elements (24 species) across our 75 metal-poor stars. Points are colored by effective temperature $T_\mathrm{eff}$, with chemically unusual stars highlighted and median errors shown in the top left of each panel. Abundances of halo stars from the Stellar Abundances for Galactic Archaeology (SAGA) database are underplotted in gray sudaStellarAbundancesGalactic2008. $\mathrm{[C / Fe]}$ is corrected for evolutionary effects based on placcoCARBONENHANCEDMETALPOORSTAR2014. The chemical abundances we derive are consistent with expected ranges for Milky Way halo stars. The complete table of chemical abundances will be made available upon publication.
• Figure 4: The original XGBoost selection remain accurate, even down to low metallicities. Shown is a comparison of $[\mathrm{M/H}]$ estimates and $[\mathrm{Fe/H}]$ measurements, colored by $T_\mathrm{eff}$. Left: direct comparison; $[\mathrm{Fe/H}]$ from our work versus $[\mathrm{M/H}]$ estimates from andraeRobustDatadrivenMetallicities2023. Right: differences between estimations (XG - our results). The selection cut applied to the catalog for observations is overplotted in blue. Both plots show minimal scatter for the majority of the stars, with outliers driven by differences in effective temperature. Metal-poor stars from the SAGA database are also underplotted, which are consistently labelled as metal-poor stars.