Mass ratio, the hills mechanism, and the galactic centre S-stars

TL;DR

This paper investigates the origins of S-stars near the galactic centre, examining binary disruption biases and tidal disruption effects to explain their lower masses compared to disk stars.

Contribution

It proposes explanations for the mass discrepancy of S-stars, including binary disruption bias and selective tidal disruption, enhancing understanding of their formation.

Findings

Binary disruption bias favors lower-mass S-stars.

Tidal disruptions have not significantly altered S-star mass distribution.

The binary disruption bias is a plausible explanation for the observed S-star properties.

Abstract

The Galactic centre contains several young populations within its central parsec: a disk between $\sim$0.05 and 0.5 pc from the centre, and the isotropic S-star cluster extending an order of magnitude further inwards in radius. Recent observations (i.e. spectroscopy and hypervelocity stars) suggest that some S-stars originate in the disk. In particular, the S-stars may be remnants of tidally disrupted disk binaries. However, there is an apparent inconsistency in this scenario: the disk contains massive O and Wolf--Rayet stars while the S-stars are lower mass, B stars. We explore two different explanations for this apparent discrepancy: (i) a built-in bias in binary disruptions, where the primary star remains closer in energy to the centre-of-mass orbit than the secondary and (ii) selective tidal disruption of massive stars within the S-star cluster. The first explanation is plausible. On the other hand, tidal disruptions have not strongly affected the mass distribution of the S-stars over the last several Myr.