Nearby high-speed stars in Gaia DR2

TL;DR

This study analyzes high-speed stars in Gaia DR2 to distinguish between bound and unbound stars, revealing that tangential velocity is a more effective indicator of unbound stars than radial velocity alone.

Contribution

Develops a novel method to evaluate whether high-speed stars are bound or unbound, demonstrating the importance of tangential velocity in identifying unbound stars using Gaia data.

Findings

Only two stars are likely unbound hyper-runaways.

Radial velocity alone is a poor indicator of unbound stars.

Tangential velocity effectively identifies nearby unbound stars.

Abstract

We investigate the nature of nearby (10-15 kpc) high-speed stars in the Gaia DR2 archive identified on the basis of parallax, proper motion and radial velocity. Together with a consideration of their kinematic, orbital, and photometric properties, we develop a novel strategy for evaluating whether high speed stars are statistical outliers of the bound population or unbound stars capable of escaping the Galaxy. Out of roughly 1.5 million stars with radial velocities, proper motions, and 5-sigma parallaxes, we identify just over 100 high-speed stars. Of these, only two have a nearly 100% chance of being unbound, with indication that they are not just bound outliers; both are likely hyper-runaway stars. The rest of the high speed stars are likely statistical outliers. We use the sample of high-speed stars to demonstrate that radial velocity alone provides a poor discriminant of nearby, unbound stars. However, nearby, unbound stars are efficiently identified from the tangential velocity, using just parallax and proper motion. Within the full Gaia DR2 archive of stars with 5-sigma parallax and proper motion but no radial velocity, we identify a sample of 19 with speeds significantly larger than the local escape speed of the Milky Way based on tangential motion alone.