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Proper Motions and Trajectories for 16 Extreme Runaway and Hypervelocity Stars

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 3-sigma 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 ejections from the disk. We also measure a significant proper motion for the unbound sdO star US 708. Its 1,000 km/s 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.