Properties of black hole-star binaries formed in $N$-body simulations of massive star clusters: implications for Gaia black holes

TL;DR

This study uses N-body simulations of massive star clusters to analyze the formation and properties of black hole-star binaries, revealing diverse systems and their potential observability with Gaia, including dynamically formed and primordial binaries.

Contribution

It provides new insights into the formation pathways and characteristics of BH-star binaries in dense clusters, especially those detectable by Gaia, through extensive simulations across various initial conditions.

Findings

Diverse BH-star binaries are produced with extreme orbital properties.

20% of ejected BH-MS binaries are formed dynamically, others are primordial.

Dynamically formed binaries tend to have more massive black holes and lower-mass companions.

Abstract

We investigate black hole-star binaries formed in $N$-body simulations of massive, dense star clusters. We simulate 32 clusters with varying initial masses ($10^{4}~\rm M_{\odot}$ to $10^{6}~\rm M_{\odot}$), densities ($1200~\rm M_{\odot}~pc^{-3}$ to $10^{5}~\rm M_{\odot}~pc^{-3}$), and metallicities $(Z = 0.01,~0.001,~0.0001)$. Our results reveal that star clusters produce a diverse range of BH-star binaries, with dynamical interactions leading to extreme systems characterised by large orbital separations and high black hole masses. Of the ejected BH-main sequence (BH-MS) binaries, $20\%$ form dynamically, while the rest originate from the primordial binary population initially present in the cluster. Ejected BH-MS binaries that are dynamically formed have more massive black holes, lower-mass stellar companions, and over half are in a hierarchical triple system. All unbound BH-giant star (BH-GS) binaries were ejected as BH-MS binaries and evolved into the BH-GS phase outside the cluster. Due to their lower-mass companions, most dynamically formed binaries do not evolve into BH-GS systems within a Hubble time. Consequently, only 2 of the 35 ejected BH-GS binaries are dynamically formed. We explore the formation pathways of Gaia-like systems, identifying two Gaia BH1-like binaries that formed through dynamical interactions, and two Gaia BH2-like systems with a primordial origin. We did not find any system resembling Gaia BH3, which may however be attributed to the limited sample size of our simulations.