Gaia EDR3 in 6D: Searching for unbound stars in the Galaxy

Tommaso Marchetti

Published 2020-12-03Version 1

The early third data release (EDR3) of the European Space Agency satellite Gaia provides the full astrometric solution (coordinates, parallax, and proper motions) for ~1.47 billion sources in our Milky Way, based on a total of 34 months of observations. The combination of Gaia DR2 radial velocities with the new, more precise, and accurate astrometry provided by Gaia EDR3 is the best dataset available to search for the fastest nearby stars in our Galaxy. We compute the velocity distribution of ~7 million stars with full phase space information and precise parallaxes, to investigate the high-velocity tail of the velocity distribution of stars in the Milky Way. We discuss the impact of adopting a parallax zero point correction. We release a catalogue with distances, total velocities, and corresponding uncertainties for all the stars considered in our analysis, available at https://sites.google.com/view/tmarchetti/research . By applying quality cuts on the Gaia astrometry and radial velocities, we identify a clean subset of 99 stars with a probability Pub >50% to be unbound from our Galaxy. 17 of these have Pub >80% and are our best candidates. We propagate these stars in the Galactic potential to characterize their orbits. We find that 10 stars are consistent with being ejected from the Galactic disk, and are possible hyper runaway star candidates. The other 7 stars are not consistent with coming from a known star-forming region, so an extragalactic origin is preferred. We do not find any unbound hypervelocity star coming from the Galactic Centre, in agreement with model predictions.