Discovery of an $[Fe/H]\sim -4.8$ Star in $Gaia$ XP Spectra

Abstract

We report on the discovery of GDR3_526285 ($Gaia$ DR3 Source ID 5262850721755411072), a star with $[Fe/H]=-4.82\pm 0.25$ and one of the lowest metal ($\text{atomic number}>2$) mass fractions ever found ($Z_{GDR3\_526585}\lesssim 1.0\times 10^{-6}$). We first identified it as an ultra metal-poor (UMP; $[Fe/H]<-4$) red giant-branch (RGB) star candidate in the $Gaia$ BP/RP (XP) spectro-photometric catalog ($Gaia$ $G$ magnitude $\approx$15). A combination of multi-band photometry and high-resolution spectroscopic analysis under local thermodynamic equilibrium confirmed the status of GDR3_526285 as a distant ($\approx$24 kpc from the Sun) RGB star ($T_{\mathrm{eff}}=4596\mathrm{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 ($\hat{I}”[C/Fe]=+0.68$) brings the nominal carbon-to-iron ratio upper limit to $[C/Fe{]}_{\mathrm{cor}}<+1.18$. 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 an useful "treasure map" for finding additional UMP stars.

Accepted to ApJL