The properties of hypervelocity stars and S-stars originating from an eccentric disc around a supermassive black hole

TL;DR

This study models the origin of hypervelocity stars and S-stars from an eccentric stellar disc around a supermassive black hole, using N-body simulations to explain their observed properties and anisotropies.

Contribution

It introduces a dynamical model where a thin eccentric disc produces hypervelocity and S-stars through binary disruption, explaining their origins and kinematic features.

Findings

Hypervelocity stars are produced via binary disruption near the SMBH.

The model explains the anisotropic distribution of hypervelocity stars.

Tightly bound S-stars originate from the same dynamical process.

Abstract

Hypervelocity stars (HVSs) that are observed in the Galactic halo, are believed to be accelerated to large velocities by a process of tidal disruption of binary stars passing close to a supermassive black hole (SMBH) which resides in the center of the Galaxy. It is, however, still unclear, where these relatively young stars were born and which dynamical process pushed them to nearly radial orbits around the SMBH. In this paper we investigate the possibility that the young binaries originated from a thin eccentric disc, similar to the one observed in the Galactic center nowadays. By means of direct N-body simulations, we follow the dynamical evolution of an initially thin and eccentric disc of stars with a 100% binary fraction orbiting around the SMBH. Such a configuration leads to Kozai-Lidov oscillations of orbital elements, bringing considerable amount of binaries to close vicinity of the black hole. Subsequent tidal disruption of these binaries accelerates one of their component to velocities well above the escape velocity from the SMBH while the second component becomes tightly bound to the SMBH. We describe the main kinematic properties of the escaping and tightly bound stars within our model and compare them qualitatively to the properties of the observed HVSs and S-stars, respectively. The most prominent feature is a strong anisotropy in the directions of the escaping stars which is observed for the Galactic HVSs but not explained yet.