Kavli Affiliate: Anna Frebel
| Summary:
We report on the discovery of GDR3_526285 ($Gaia$ DR3 Source ID 5262850721755411072), a star with $rm[Fe/H] = -4.82 pm 0.25$ and one of the lowest metal ($textatomic number > 2$) mass fractions ever found ($Z_rm GDR3_526585 lesssim 1.0 times 10^-6$). We first identified it as an ultra metal-poor (UMP; $rm[Fe/H] < -4$) 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_rm eff = 4596,rm K$, $log g = 0.88$) in the Milky Way’s outer halo. We obtain only an upper limit for the carbon abundance of $rm[C/H] < -4.32$, resulting in $rm[C/Fe] < +0.50$. A correction for the evolutionary carbon depletion ($Δrm[C/Fe] = +0.68$) brings the nominal carbon-to-iron ratio upper limit to $rm[C/Fe]_rm cor < +1.18$. 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.
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