A Successful Targeted Search for Hypervelocity Stars

Abstract

Hypervelocity stars (HVSs) travel with velocities so extreme that dynamical ejection from a massive black hole is their only suggested origin. Following the discovery of the first HVS by Brown and collaborators, we have undertaken a dedicated survey for more HVSs in the Galactic halo and present here the resulting discovery of two new HVSs: SDSS J091301.0+305120 and SDSS J091759.5+672238, traveling with Galactic rest-frame velocities of at least +558 +/- 12 and +638 +/- 12 km s -1, respectively. Assuming the HVSs are B8 main-sequence stars, they are at distances of ~75 and ~55 kpc, respectively, and have travel times from the Galactic center consistent with their lifetimes.The existence of two B8 HVSs in our 1900 deg 2 survey, combined with the Yu & Tremaine HVS rate estimates, is consistent with HVSs drawn from a standard initial mass function but inconsistent with HVS drawn from a truncated mass function like the one in the top-heavy Arches cluster. The travel times of the five currently known HVSs provide no evidence for a burst of HVSs from a major infall event at the Galactic center in the last ~160 Myr.