Discovery of an Unbound Hypervelocity Star in the Milky Way Halo

We have discovered a star, SDSS J090745.0+024507, leaving the Galaxy with a heliocentric radial velocity of 853 ± 12 km s^-1, the largest velocity ever observed in the Milky Way halo. The star is either a hot blue horizontal-branch star or a B9 main-sequence star with a heliocentric distance of 39 or 71 kpc, respectively. Corrected for the solar reflex motion and to the local standard of rest, the Galactic rest-frame velocity is 709 km s^-1. We suggest that this star is the first example of a hypervelocity star ejected from the Galactic center, as predicted by Hills and later discussed by Yu & Tremaine. The star's radial velocity vector points 174° from the Galactic center. The star has [Fe/H] ~ 0, consistent with a Galactic center origin, and a travel time of ≲80 Myr from the Galactic center, consistent with its stellar lifetime. If the star is indeed traveling from the Galactic center, it should have a proper motion of ~0.3 mas yr^-1 observable with the Space Interferometry Mission or the Global Astrometric Interferometer for Astrophysics. Identifying additional hypervelocity stars throughout the halo will constrain the production rate history of hypervelocity stars at the Galactic center.