Demographics of neutron stars in young massive and open clusters

TL;DR

This study uses advanced N-body simulations to explore how neutron stars form, evolve, and merge in young star clusters, revealing their distribution, retention, and merger rates, which are lower than observed gravitational wave events.

Contribution

First application of high-precision N-body simulations to analyze neutron star dynamics and merger rates in young massive and open clusters with varying initial conditions.

Findings

Most neutron stars escape clusters due to high natal kicks.

Neutron star distribution is influenced by black hole content in clusters.

Predicted neutron star merger rates are much lower than current LIGO/Virgo observations.

Abstract

Star clusters appear to be the ideal environment for the assembly of neutron star-neutron star (NS-NS) and black hole-neutron star (BH-NS) binaries. These binaries are among the most interesting astrophysical objects, being potential sources of gravitational waves (GWs) and gamma-ray bursts. We use for the first time high-precision N-body simulations of young massive and open clusters to study the origin and dynamical evolution of NSs, within clusters with different initial masses, metallicities, primordial binary fractions, and prescriptions for the compact object natal kicks at birth. We find that the radial profile of NSs is shaped by the BH content of the cluster, which partially quenches the NS segregation due to the BH-burning process. This leaves most of the NSs out of the densest cluster regions, where NS-NS and BH-NS binaries could potentially form. Due to a large velocity kick that they receive at birth, most of the NSs escape the host clusters, with the bulk of their retained population made up of NSs of $\sim 1.3$ M$_\odot$ coming from the electron-capture supernova process. The details of the primordial binary fraction and pairing can smear out this trend. Finally, we find that a subset of our models produce NS-NS mergers, leading to a rate of $\sim 0.01$--$0.1$ Gpc$^{-3}$ yr$^{-1}$ in the local Universe, and compute an upper limit of $\sim 3\times 10^{-2}$--$3\times 10^{-3}$ Gpc$^{-3}$ yr$^{-1}$ for the BH-NS merger rate. Our estimates are several orders of magnitude smaller than the current empirical merger rate from LIGO/Virgo, in agreement with the recent rate estimates for old globular clusters.