Double progenitor origin of the S-star cluster

TL;DR

This study explores the origins of the S-star cluster near the Galactic Centre, demonstrating that a combined binary injection from both the nuclear star cluster and the clockwise disk best explains observed properties and hypervelocity stars.

Contribution

The paper introduces a model showing that both the nuclear star cluster and the clockwise disk contribute to the S-star cluster's formation, resolving previous conflicting theories.

Findings

Combined binary injection explains all observations.

~90% of hypervelocity stars originate from the CWD.

Main-sequence S-stars come from the CWD, evolved S-stars from the NSC.

Abstract

The origin of the cluster of S-stars located in the Galactic Centre is tied to the supermassive black hole Sagittarius A*, but exactly how is still debated. In this paper, we investigate whether the Hills mechanism can simultaneously reproduce both the S-star cluster's properties and the observed number of hypervelocity stars. To do so, we forward-model the capture and disruption of binary stars originating from the nuclear star cluster (NSC) and the clockwise disk (CWD). We find that the ratio of evolved to main-sequence S-stars is highly sensitive to the origin of the binaries, and that neither the injection of binaries from the CWD nor from the NSC exclusively can reproduce all observations. However, when considering the injection of binaries from both locations, we are able to reproduce all the observations simultaneously, including the number of observed hypervelocity stars, the evolutionary stage of the S-stars, their luminosity function and the distribution of their semi-major axes. The implications of our findings include that ~90% of hypervelocity stars ejected over the past ~10 Myr should originate from the CWD, that the main-sequence S-stars originated in the CWD, and that the evolved S-stars originated in an old stellar population such as the NSC.