SEGUE-2 Limits on Metal-Rich Old-Population Hypervelocity Stars In the Galactic Halo

TL;DR

This study sets new, more stringent limits on the number of metal-rich old-population hypervelocity stars ejected from the Galactic center, constraining ejection mechanisms and the GC mass function.

Contribution

It provides the first comprehensive limits on metal-rich old-population hypervelocity stars, improving previous constraints by a factor of 3-10.

Findings

Limits are 3-10 times more stringent than previous reports.

No more than 30 times more metal-rich F/G stars ejected than B-stars.

Ejection mechanism must be about 5.5 times more efficient for B-stars.

Abstract

We present new limits on the ejection of metal-rich old-population hypervelocity stars from the Galactic center (GC) as probed by the SEGUE-2 survey. Our limits are a factor of 3-10 more stringent than previously reported, depending on stellar type. Compared to the known population of B-star ejectees, there can be no more than 30 times more metal-rich old-population F/G stars ejected from the GC. Because B stars comprise a tiny fraction of a normal stellar population, this places significant limits on a combination of the GC mass function and the ejection mechanism for hypervelocity stars. In the presence of a normal GC mass function, our results require an ejection mechanism that is about 5.5 times more efficient at ejecting B-stars compared to low-mass F/G stars.