Kavli Affiliate: Huawei Zhang
| Summary:
We report the discovery of DESI-HVS1, a hypervelocity star (HVS) candidate identified from DESI DR1 spectroscopy and Gaia DR3 astrometry. DESI-HVS1 is an old, low-mass, metal-poor F-type star with a mass of $0.8,M_odot$, an age of $sim14.1$~Gyr, and $mathrm[Fe/H]=-1.6$. It is located at a heliocentric distance of $3.77^+0.39_-0.36$~kpc and has a Galactocentric total velocity of $523^+46_-47,mathrmkm,s^-1$, marginally exceeding the local escape speed, corresponding to an unbound probability of $P_rm ub sim 50%$. Backward orbit integrations show that DESI-HVS1 had a closest approach to the Galactic center (GC) of $0.40^+0.23_-0.11,mathrmkpc$, with a velocity of $682^+22_-35,mathrmkm,s^-1$ and a flight time of $12.89^+0.92_-0.74,mathrmMyr$. The reconstructed orbit exhibits a clear perigalactic turning point and only a single crossing of the Galactic midplane ($P_rm cross > 0.95$). These properties suggest that DESI-HVS1 is most naturally explained by the Hills mechanism, although alternative scenarios cannot be entirely ruled out. Its discovery provides the first strong evidence for an old, low-mass HVS candidate consistent with a GC origin, indicating that the apparent dominance of young, massive GC-origin HVSs is likely a consequence of observational selection effects.
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