New Nearby Hypervelocity Stars and Their Spatial Distribution from Gaia DR2

Abstract

Based on about 4500 large tangential velocity (V _tan > 0.75V _esc) with high-precision proper motions and 5σ parallaxes in Gaia data release 2 (DR2) 5D information derived from parallax and proper motion, we identify more than 600 high-velocity stars with a 50% unbound probability. Of these, 28 nearby (less than 6 kpc) late-type hypervelocity stars (HVSs) with over a 99% possibility of being unbound are discovered. In order to search for the unbound stars from the full Gaia DR2 6D phase space information derived from the parallax, proper motion, and radial velocity, we also identify 28 stars from the total velocity (V _gc > 0.75V _esc) that have probabilities greater than 50% of being unbound from the Galaxy. Of these, only three have a nearly 99% probability of being unbound. In the whole HVSs subsample, there is 12 sources reported by other surveys. We study the spatial distribution of angular positions and angular separation of HVSs. We find the unbound HVSs are spatially anisotropic, which is most significant in the Galactic longitude at more than a 3σ level, and lower unbound probability HVSs are systematically more isotropic. The spatial distribution can reflect the origin of HVSs and we discuss the possible origin link with the anisotropy.