Tidal breakup of triple stars in the Galactic Centre

TL;DR

This paper investigates how the breakup of stellar triples near the Galactic Centre's black hole can produce hypervelocity stars and binaries, but finds this mechanism unlikely to account for all observed hypervelocity stars.

Contribution

It provides a numerical analysis showing that triple disruptions rarely produce hypervelocity binaries, suggesting other mechanisms are responsible for such stars.

Findings

Most outcomes are single hypervelocity stars or stars bound to the black hole.

Hypervelocity binary production probability is less than 1%.

Ejection rates are too low to explain observed short-lived hypervelocity stars.

Abstract

The last decade has seen the detection of fast moving stars in the Galactic halo, the so-called hypervelocity stars (HVSs). While the bulk of this population is likely the result of a close encounter between a stellar binary and the supermassive black hole (MBH) in the Galactic Centre (GC), other mechanims may contribute fast stars to the sample. Few observed HVSs show apparent ages which are shorter than the flight time from the GC, thereby making the binary disruption scenario unlikely. These stars may be the result of the breakup of a stellar triple in the GC which led to the ejection of a hypervelocity binary (HVB). If such binary evolves into a blue straggler star due to internal processes after ejection, a rejuvenation is possible that make the star appear younger once detected in the halo. A triple disruption may also be responsible for the presence of HVBs, of which one candidate has now been observed. We present a numerical study of triple disruptions by the MBH in the GC and find that the most likely outcomes are the production of single HVSs and single/binary stars bound to the MBH, while the production of HVBs has a probability $\lesssim 1\%$ regardless of the initial parameters. Assuming a triple fraction of $\approx 10\%$ results in an ejection rate of $\lesssim 1\ \mathrm{Gyr}^{-1}$, insufficient to explain the sample of HVSs with lifetimes shorter than their flight time. We conclude that alternative mechanisms are responsible for the origin of such objects and HVBs in general.