Discovery of an Unbound Hypervelocity Star in the Milky Way Halo

We have discovered a star, SDSS J090745.0+024507, leaving the Galaxy with aheliocentric radial velocity of +853+-12 km/s, the largest velocity everobserved in the Milky Way halo. The star is either a hot blue horizontal branchstar or a B9 main sequence star with a heliocentric distance ~55 kpc. Correctedfor the solar reflex motion and to the local standard of rest, the Galacticrest-frame velocity is +709 km/s. Because its radial velocity vector points 173.8 deg from the Galactic center,we suggest that this star is the first example of a hyper-velocity star ejectedfrom the Galactic center as predicted by Hills and later discussed by Yu &Tremaine. The star has [Fe/H]~0, consistent with a Galactic center origin, anda travel time of <80 Myr from the Galactic center, consistent with its stellarlifetime. If the star is indeed traveling from the Galactic center, it shouldhave a proper motion of 0.3 mas/yr observable with GAIA. Identifying additionalhyper-velocity stars throughout the halo will constrain the production ratehistory of hyper-velocity stars at the Galactic center.Comment: 4 pages, submitted to ApJ Letter