Gaia and the Galactic Center Origin of Hypervelocity Stars
  We use new Gaia measurements to explore the origin of the highest velocity
stars in the Hypervelocity Star Survey. The measurements reveal a clear pattern
in the B-type stars. Halo stars dominate the sample at speeds about 100 km/s
below Galactic escape velocity. Disk runaway stars have speeds up to 100 km/s
above Galactic escape velocity, but most disk runaways are bound. Stars with
speeds about 100 km/s above Galactic escape velocity originate from the
Galactic center. Two bound stars may also originate from the Galactic center.
Future Gaia measurements will enable a large, clean sample of Galactic center
ejections for measuring the massive black hole ejection rate of hypervelocity
stars, and for constraining the mass distribution of the Milky Way dark matter
halo.
