The nature of hypervelocity stars as inferred from their galactic trajectories

TL;DR

This study models the trajectories of hypervelocity stars to determine their origins, suggesting most are massive main sequence stars ejected from the Galactic Centre through binary breakup, with implications for understanding the SMBH environment.

Contribution

It provides the first detailed trajectory analysis of hypervelocity stars, linking their origins to binary interactions near the Galactic Centre and distinguishing halo objects from ejected stars.

Findings

Eight of twelve HVSs are bound to the Galaxy.

Bound stars have an orbital period of about 2 Gyr.

Most HVSs are massive main sequence stars, not blue horizontal branch stars.

Abstract

We have computed the galactic trajectories of twelve hypervelocity stars (HVSs) under the assumption that they originated in the Galactic Centre. We show that eight of these twelve stars are bound to the Galaxy. We consider the subsequent trajectories of the bound stars to compute their characteristic orbital period, which is 2 Gyr. All eight bound stars are moving away from the centre of the Galaxy, which implies that the stars' lifetimes are less than 2 Gyr. We thus infer that the observed HVSs are massive main sequence stars, rather than blue horizontal branch stars. The observations suggest that blue HVSs are ejected from the Galactic Centre roughly every 15 Myr. This is consistent with the observed population of blue stars in extremely tight orbits round the central super-massive black hole (SMBH), the so-called S-stars, if we assume that the HVSs are produced by the breakup of binaries. One of the stars in such a binary is ejected at high velocities to form a HVS; the other remains bound to the SMBH as an S-star. We further show that the one high-velocity system observed to be moving towards the Galactic Centre, SDSS J172226.55+594155.9, could not have originated in the Galactic Centre; rather, we identify it as a halo object.