Fastest stars in the Galaxy with Gaia DR2

Of these, 20 stars have probabilities greater than 80 per cent of being unbound from the Galaxy. On this latter subsample, we perform orbit integration to characterize the stars' orbital parameter distributions. As expected given the relatively small sample size of bright stars, we find no hypervelocity star candidates, stars that are moving on orbits consistent with coming from the Galactic Centre.

Instead, we find seven hyperrunaway star candidates, coming from the Galactic disc. Surprisingly, the remaining 13 unbound stars cannot be traced back to the Galaxy, including two of the fastest stars (around 700km/s). If conformed, these may constitute the tip of the iceberg of a large extragalactic population or the extreme velocity tail of stellar streams.