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

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) M-circle dot to 10(6) M-circle dot), densities (1200 M(circle dot)pc(-3) to 10(5) M-circle dot 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 characterized by large orbital separations and high black hole masses. Of the ejected BH-main sequence (BH-MS) binaries, 20 per cent 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.