Three-body encounters in the Galactic Centre: the origin of the hypervelocity star SDSS J090745.0+024507

In the late 1980s Hills predicted that runaway stars could be accelerated to velocities greater than 1000 km s−1 by dynamical encounters with the supermassive black hole (SMBH) in the Galactic Centre. The recently discovered hypervelocity star SDSS J090745.0+024507 (hereafter the HVS) is escaping the Galaxy at high speed and could be the first object in this class. With the measured radial velocity and the estimated distance to the HVS, we trace back its trajectory in the Galactic potential. Assuming it was ejected from the Centre, we find that a ∼2 mas yr−1 proper motion is necessary for the star to have come within a few parsecs of the SMBH. We perform three-body scattering experiments to constrain the progenitor encounter that accelerated the HVS. As proposed recently by Yu & Tremaine, we consider the tidal disruption of binary systems by the SMBH and the encounter between a star and a binary black hole, as well as an alternative scenario involving intermediate-mass black holes. We find that the tidal disruption of a stellar binary ejects stars with a larger velocity compared to the encounter between a single star and a binary black hole, but has a somewhat smaller ejection rate due to the greater availability of single stars.