Discovery of an Fe/H ∼ −4.8 Star in Gaia XP Spectra*

Abstract We report on the discovery of GDR3₅26285 (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 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 (ΔC/Fe = +0. 68) brings the nominal carbon-to-iron ratio upper limit to C/Fe cor < +1. 18. Given its extraordinarily low C/H, GDR3₅26285 likely formed from gas cooled via dust grains rather than fine structure line cooling. The kinematics of GDR3₅26285 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.