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We measure proper motions with the Hubble Space Telescope for 16 extremeradial velocity stars, mostly unbound B stars in the Milky Way halo. Twelve ofthese stars have proper motions statistically consistent with zero, and thushave radial trajectories statistically consistent with a Galactic center"hypervelocity star" origin. The trajectory of HE 0437-5439 is consistent withboth Milky Way and Large Magellanic Cloud origins. A Galactic center origin isexcluded at 3-sigma confidence for two of the lowest radial velocity stars inour sample, however. These two stars are probable disk runaways and provideevidence for ~500 km/s ejections from the disk. We also measure a significantproper motion for the unbound sdO star US 708. Its 1,000 km/s motion is in sometension with proposed supernova ejection models, but can be explained if US 708was ejected from the stellar halo. In the future, we expect Gaia will betterconstrain the origin of these remarkable unbound stars.

Proper Motions and Trajectories for 16 Extreme Runaway and Hypervelocity Stars