MMT Hypervelocity Star Survey

TL;DR

This survey identified new hypervelocity stars likely ejected from the Galactic center, increasing the known population and providing insights into stellar ejection mechanisms and the Galactic center's stellar mass function.

Contribution

It presents a new survey method for detecting unbound hypervelocity stars and reports the discovery of 10 new HVSs, doubling the known sample and exploring their properties.

Findings

Discovered 6 unbound HVSs exceeding conservative escape velocities.

Identified 4 possibly unbound HVSs exceeding lower escape velocities.

Increased the known HVS population by 60-100%.

Abstract

We describe a new survey for unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their most likely origin. We investigate the possible contribution of unbound runaway stars, and show that the physical properties of binaries constrain low mass runaways to bound velocities. We measure radial velocities for HVS candidates with the colors of early A-type and late B-type stars. We report the discovery of 6 unbound HVSs with velocities and distances exceeding the conservative escape velocity estimate of Kenyon and collaborators. We additionally report 4 possibly unbound HVSs with velocities and distances exceeding the lower escape velocity estimate of Xue and collaborators. These discoveries increase the number of known HVSs by 60%-100%. Other survey objects include 19 newly identified z~2.4 quasars. One of the HVSs may be a horizontal branch star, consistent with the number of evolved HVSs predicted by Galactic center ejection models. Finding more evolved HVSs will one day allow a probe of the low-mass regime of HVSs and will constrain the mass function of stars in the Galactic center.