Tidal breakup of quadruple stars in the Galactic Centre

TL;DR

This study investigates the tidal disruption of quadruple star systems in the Galactic Centre by the supermassive black hole, assessing their potential to produce hypervelocity binaries and comparing this mechanism to other proposed origins.

Contribution

The paper provides a numerical analysis of 2+2 quadruple star disruptions by the MBH, estimating their low probability of producing hypervelocity binaries compared to other mechanisms.

Findings

HVBs produced by quadruple disruptions have a probability of less than 4%.

The ejection rate of HVBs from quadruple disruptions is about 1 per billion years.

Alternative mechanisms likely explain the origin of observed hypervelocity binaries.

Abstract

The most likely origin of hypervelocity stars (HVSs) is the tidal disruption of a binary star by the supermassive black hole (MBH) in the Galactic Centre (GC). However, HE0437-5439, a $9$ M$_\odot$ B-type main-sequence star moving with a heliocentric radial velocity of about $720$ km s$^{-1}$ at a distance of $\sim 60$ kpc, and the recent discovered hypervelocity binary candidate (HVB), traveling at $\sim 570$ km s$^{-1}$, challenge this standard scenario. Recently, Fragione & Gualandris (2018) have demonstrated that the tidal breakup of a triple star leads to an insufficient rate. Observations show that quadruple stars made up of two binaries orbiting their common center of mass (the so-called 2+2 quadruples) are $\approx 4\%$ of the stars in the solar neighborhood. Although rarer than triples, 2+2 quadruple stars may have a role in ejecting HVBs as due to their larger energy reservoir. We present a numerical study of 2+2 quadruple disruptions by the MBH in the GC and find that the production of HVBs has a probability $\lesssim 2-4\%$, which translates into an ejection rate of $\lesssim 1$ Gyr$^{-1}$, comparable to the triple disruption scenario. Given the low ejection rate, we suggest that alternative mechanisms are responsible for the origin of HVBs, as the ejection from the interaction of a young star cluster with the MBH in the GC and the origin in the Large Magellanic Cloud.