Compact Binary Formation in Open Star Clusters I: High Formation Efficiency of Gaia BHs and Their Multiplicities

TL;DR

This study shows that Gaia black hole binaries can form efficiently through dynamical interactions in open clusters, with many having tertiary companions, and estimates a large population in the Milky Way.

Contribution

It demonstrates that Gaia BHs are likely formed via dynamical capture in open clusters, not just isolated binary evolution, and provides population estimates.

Findings

Gaia BH formation efficiency in open clusters is 3 orders of magnitude higher than in isolated binaries.

Most Gaia BHs in open clusters are part of triple star systems.

Estimated 10^4 to 10^5 Gaia BHs in the Milky Way disk.

Abstract

Gaia BHs, black hole (BH) binaries discovered from database of an astrometric telescope Gaia, pose a question to the standard binary evolution model. We have assessed if Gaia BHs can be formed through dynamical capture in open clusters rather than through isolated binary evolution. We have performed gravitational $N$-body simulations of $100$ open clusters with $10^5 M_\odot$ in total for each metallicity $Z=0.02$, $0.01$, and $0.005$. We have discovered one Gaia BH-like binary escaping from an open cluster, and found that the formation efficiency of Gaia BHs in open clusters ($\sim 10^{-5} M_\odot^{-1}$) is larger than in isolated binaries ($\sim 10^{-8} M_\odot^{-1}$) by 3 orders of magnitude. The Gaia BH-like binary is the inner binary of a triple star system. Gaia BHs can have tertiary stars frequently, if they are formed in open clusters. Combining additional $N$-body simulations with 8000 open clusters with $8 \times 10^6 M_\odot$, we have estimated the number of Gaia BHs in the Milky Way disk to $10^4 - 10^5$ (depending on the definitions of Gaia BHs), large enough for the number of Gaia BHs discovered so far. Our results indicate that the discoveries of Gaia BHs do not request the reconstruction of the standard binary evolution model, and that Gaia BHs are a probe for the dynamics of open clusters already evaporated.