The Origin of HVS17, an Unbound Main Sequence B Star at 50 kpc
  We analyze Keck ESI spectroscopy of HVS17, a B-type star traveling with a
Galactic rest frame radial velocity of +445 km/s in the outer halo of the Milky
Way. HVS17 has the projected rotation of a main sequence B star and is
chemically peculiar, with solar iron abundance and sub-solar alpha abundance.
Comparing measured T_eff and logg with stellar evolution tracks implies that
HVS17 is a 3.91 +-0.09 Msun, 153 +-9 Myr old star at a Galactocentric distance
of r=48.5 +-4.6 kpc. The time between its formation and ejection significantly
exceeds 10 Myr and thus is difficult to reconcile with any Galactic disk
runaway scenario involving massive stars. The observations are consistent, on
the other hand, with a hypervelocity star ejection from the Galactic center. We
show that Gaia proper motion measurements will easily discriminate between a
disk and Galactic center origin, thus allowing us to use HVS17 as a test
particle to probe the shape of the Milky Way's dark matter halo.
