We measure proper motions with the Hubble Space Telescope for 16 extreme radial velocity stars, mostly unbound B stars in the Milky Way halo. Twelve of these stars have proper motions statistically consistent with zero, and thus have radial trajectories statistically consistent with a Galactic center “hypervelocity star” origin. The trajectory of HE 0437–5439 is consistent with both Milky Way and Large Magellanic Cloud origins. A Galactic center origin is excluded at 3s confidence for two of the lowest radial velocity stars in our sample, however. These two stars are probable disk runaways and provide evidence for ~500 km s-1 ejections from the disk. We also measure a significant proper motion for the unbound sdO star US 708. Its 1000 km s-1 motion is in some tension with proposed supernova ejection models, but can be explained if US 708 was ejected from the stellar halo. In the future, we expect Gaia will better constrain the origin of these remarkable unbound stars. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, and from the data archive at STScI, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
