Abstract

The recent second data release (DR2) of the ESA satellite Gaia gives us the first opportunity to search for high velocity stars in the largest stellar catalogue ever produced. Categories of great interest are runaway stars, high velocity halo stars, and hypervelocity stars (HVSs). In this talk I will present our results on the high velocity tail of the velocity distribution of Milky Way stars, using the subset of about 7 million stars with a radial velocity measurement from Gaia DR2 . This study reveals the presence of a population of about 20 unbound stars, half of them with a possible extra-Galactic origin. I will also present ongoing results on the data mining routine we specifically developed to search for the fastest stars, HVSs, in the full (approximately 1.3 billion sources) Gaia DR2 catalogue. Our published estimates show that hundreds to thousands of HVSs are expected to be contained in the full Gaia catalogue, but their identification is not trivial because of the missing radial velocity information, which needs to be obtained with a spectroscopic follow-up. I will conclude presenting HVSs as a powerful probe to map the dark halo of the Milky Way: a population of ~100 Gaia HVSs could be used to nail down the Galactic halo potential parameters with sub-percent precision.