Chemical abundances of a Galactic Centre ejecta discovered in DESI

Abstract

Chemical abundances in the Galactic Centre (GC) are challenging to measure because of high extinction, crowding, and distance, and are typically limited to the brightest AGB and M giants observed in the near-infrared. In contrast, hypervelocity stars (HVSs) ejected from the GC by Sgr A* carry their natal composition into accessible regions such as the Milky Way halo, making them unique probes of GC chemistry. We present high-resolution optical Magellan/MIKE spectroscopy of DESI-312, a bound HVS candidate in the halo with super-solar metallicity ($[\mathrm{Fe/H}]=0.2\pm0.1$). This sets it apart from accreted halo populations on radial orbits and is most naturally explained by GC formation and Hills-mechanism ejection. Updated isochrone fits give an age of 7.4-11.3 Gyr; together with its metallicity and kinematics, this strengthens the case for a GC origin. Korg and Turbospectrum spectral synthesis yields $[\mathrm{X/Fe}]$ for 22 elements, plus Li from an independent line-by-line NLTE analysis. As DESI-312 is near the main sequence, its C, N, and O abundances are unaltered by dredge-up; to our knowledge, these are the first such measurements of unevolved GC material. Its abundance pattern lies where the nuclear star cluster (NSC), nuclear stellar disc (NSD), and inner-bulge sequences overlap, with a Na enhancement ($[\mathrm{Na/Fe}]=+0.16^{+0.03}_{-0.04}$) more modest than in NSC giants of similar metallicity. Chemistry alone thus cannot identify its progenitor; we nevertheless favour the NSC, given its large reservoir of old, metal-rich stars and the star's kinematics. Alongside WINERED-HVS1 and the S-star S0-6/S10, whose abundances point to different progenitors, this tentatively suggests multiple progenitor populations for the S-star cluster. DESI-312 is a test case for sampling GC chemistry with ejected dwarfs at optical wavelengths, complementing near-infrared studies of giants.