Discovery of Galactic center ejected star in DESI DR1

Abstract

Hypervelocity stars (HVSs) are stars ejected from the Galactic Centre (GC) through tidal interactions with the central supermassive black hole. Formed in the immediate vicinity of Sgr~A$^\ast$, these stars are accelerated to velocities high enough to escape the GC and be observable in the Galactic halo. Using spectroscopy from the Dark Energy Spectroscopic Instrument (DESI) and astrometry from Gaia, we conducted a six-dimensional search for HVSs and identified a compelling candidate, hereafter DESI-312, whose bound trajectory can be confidently traced back to the central 2 kpc of our galaxy. The star resides in the inner halo and exhibits supersolar metallicity ([Fe/H] $= 0.27\pm 0.09$), distinct from other known stellar populations with radial orbits. Its inferred GC ejection velocity of $698^{+35}_{-27}$ is consistent with a Hills mechanism ejection, supporting an origin in the innermost regions of the Milky Way. We considered alternative origins for the star, including disk ejections from young clusters and globular clusters, but these scenarios fail to explain both its orbit and metallicity. Unlike previously identified A- and B-type HVSs, DESI-312 is a $\sim 1\,M_{\odot}$ star on the main sequence or early subgiant branch, thus enabling a detailed chemical analysis of its atmosphere and offering a rare window - unobscured by dust and crowding - into the composition of the central regions of the Galaxy.