The dominant mechanism(s) for populating the outskirts of star clusters with neutron star binaries

TL;DR

This paper investigates the mechanisms populating the outskirts of star clusters with neutron star binaries, concluding that three-body interactions, rather than black hole binaries, are the dominant process.

Contribution

It challenges the idea that black hole binaries expel neutron star binaries to cluster outskirts, showing instead that three-body interactions are primarily responsible.

Findings

Black hole binaries rarely retain neutron star binaries in clusters.

Three-body interactions are the main mechanism for neutron star binaries in outskirts.

Models match observed distributions without invoking black hole binaries.

Abstract

It has been argued that heavy binaries composed of neutron stars (NSs) and millisecond pulsars (MSPs) can end up in the outskirts of star clusters via an interaction with a massive black hole (BH) binary expelling them from the core. We argue here, however, that this mechanism will rarely account for such observed objects. Only for primary masses $\lesssim$ 100 M$_{\odot}$ and a narrow range of orbital separations should a BH-BH binary be both dynamically hard and produce a sufficiently low recoil velocity to retain the NS binary in the cluster. Hence, BH binaries are in general likely to eject NSs from clusters. We explore several alternative mechanisms that would cause NS/MSP binaries to be observed in the outskirts of their host clusters after a Hubble time. The most likely mechanism is a three-body interaction involving the NS/MSP binary and a normal star. We compare to Monte Carlo simulations of cluster evolution for the globular clusters NGC 6752 and 47 Tuc, and show that the models not only confirm that normal three-body interactions involving all stellar-mass objects are the dominant mechanism for putting NS/MSP binaries into the cluster outskirts, they also reproduce the observed NS/MSP binary radial distributions without needing to invoke the presence of a massive BH binary. Higher central densities and an episode of core-collapse can broaden the radial distributions of NSs/MSPs and NS/MSP binaries due to three-body interactions, making these clusters more likely to host NSs in the cluster outskirts.