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 (HVS) survey. The measurements reveal a clear pattern in B-type stars. Halo stars dominate the sample at speeds of similar or equal to 100 km s(-1) below Galactic escape velocity. Disk runaway stars have speeds up to similar or equal to 100 km s(-1) above Galactic escape velocity, but most disk runaways are bound. Stars with speeds greater than or similar to 100 km s(-1) 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 HVSs, and for constraining the mass distribution of the Milky Way dark matter halo.