MMT Hypervelocity Star Survey III: A Complete Survey of Faint B-type Stars in the Northern Milky Way Halo

TL;DR

This survey identified 21 unbound hypervelocity stars ejected from the Milky Way, providing new constraints on their properties, distribution, and ejection mechanisms, primarily linked to the Galactic center's black hole.

Contribution

It presents a complete spectroscopic survey of faint B-type hypervelocity stars, expanding the known sample and analyzing their origins, distribution, and ejection rates.

Findings

21 unbound HVSs identified, including 3 new discoveries.

HVSs are mainly 2.5-4 Msun main sequence stars at 50-120 kpc.

Ejection rate of 1.5e-6 per year for these stars.

Abstract

We describe our completed spectroscopic survey for unbound hypervelocity stars (HVSs) ejected from the Milky Way. Three new discoveries bring the total number of unbound HVSs to 21. We place new constraints on the nature of HVSs and on their distances using moderate resolution MMT spectroscopy. Half of the HVSs are fast rotators; they are certain 2.5-4 Msun main sequence stars at 50 - 120 kpc distances. Correcting for stellar lifetime, our survey implies that unbound 2.5-4 Msun stars are ejected from the Milky Way at a rate of 1.5e-6 /yr. The observed HVSs are likely ejected continuously over the past 200 Myr and do not share a common flight time. The anisotropic spatial distribution of HVSs on the sky remains puzzling. Southern hemisphere surveys like SkyMapper will soon allow us to map the all-sky distribution of HVSs. Future proper motion measurements with Hubble Space Telescope and Gaia will provide strong constraints on origin. All existing observations are consistent with HVS ejections from encounters with the massive black hole in the Galactic center.