2012ApJ...754L...2B

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 M_☉ 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 10⁸ yr "arrival time" between formation and ejection is difficult to reconcile with any ejection scenario involving massive stars that live for only 10⁷ yr. For comparison, we derive arrival times of 10⁷ 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 10⁷ yr of its lifetime is ruled out at the 3σ level. Together with the 10⁸ 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.