The Nature of Hypervelocity Stars and the Time Between Their Formation
  and Ejection
  We obtain Keck HIRES spectroscopy of HVS5, one of the fastest unbound stars
in the Milky Way halo. We show that HVS5 is a 3.62 +- 0.11 Msun main sequence B
star at a distance of 50 +- 5 kpc. The difference between its age and its
flight time from the Galactic center is 105 +-18(stat)+-30(sys) Myr; flight
times from locations elsewhere in the Galactic disk are similar. This 10^8 yr
`arrival time' between formation and ejection is difficult to reconcile with
any ejection scenario involving massive stars that live for only 10^7 yr. For
comparison, we derive arrival times of 10^7 yr for two unbound runaway B stars,
consistent with their disk origin where ejection results from a supernova in a
binary system or dynamical interactions between massive stars in a dense star
cluster. For HVS5, ejection during the first 10^7 yr of its lifetime is ruled
out at the 3-sigma level. Together with the 10^8 yr arrival times inferred for
three other well-studied hypervelocity stars (HVSs), these results are
consistent with a Galactic center origin for the HVSs. If the HVSs were indeed
ejected by the central black hole, then the Galactic center was forming stars
~200 Myr ago, and the progenitors of the HVSs took ~100 Myr to enter the black
hole's loss cone.
