The ejection and detectability of high- and hyper-velocity stars by compact object binaries in globular clusters

TL;DR

This study models how interactions between stars and compact object binaries in globular clusters can eject stars at high velocities, potentially contributing to the population of hypervelocity stars and informing future observations.

Contribution

It combines simulations and observations to quantify the ejection of high-velocity stars from globular clusters due to star-compact object binary interactions, a novel approach in this context.

Findings

Approximately 6300 stars ejected in last 500 Myr from GCs.

Around 839 stars have velocities exceeding 500 km/s.

Future surveys like LSST will improve detection prospects.

Abstract

The dense cores of Milky Way globular clusters (GCs) play host to a variety of dynamical encounters between stellar objects, which can accelerate stars to velocities high enough to escape the GC. The most extreme examples of these encounters are interactions between single GC stars and binaries including at least one compact object. These interactions can result in ejection velocities of up to several hundred $\mathrm{km \ s^{-1}}$, approaching or even exceeding the escape velocity of the Galaxy itself. In order to study whether these interactions contribute to the Galactic population of hypervelocity stars (stars moving faster than the Galactic escape speed), we combine Monte Carlo $N$-body GC simulations, observations of Galactic GCs, and a particle spray code to generate realistic populations of stars which have escaped from Milky Way GCs following star + compact object binary (S+COB) interactions. We find that over the last 500 Myr, S+COB interactions have likely ejected $\sim$6300 stars from Galactic GCs, of which $839_{-67}^{+70}$ have present-day velocities exceeding $500 \; \mathrm{km \ s^{-1}}$. Using mock photometric observations, we find that $290_{-23}^{+28}$ ejected stars are detectable in Gaia Data Release 3, however, only $1_{-1}^{+2}$ stars faster than $500 \; \mathrm{km \ s^{-1}}$ are detectable. Even so, we show that observational prospects in the upcoming Legacy Survey of Space and Time are more optimistic, and future detected fast extratidal GC stars will serve as a useful probe of GC cores.