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 105M⊙ 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 (∼10−5M⊙−1) is larger than in isolated binaries (∼10−8M⊙−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 8×106M⊙, we have estimated the number of Gaia BHs in the Milky Way disk to 104−105 (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.
@article{arxiv.2303.05743,
title = {Compact Binary Formation in Open Star Clusters I: High Formation Efficiency of Gaia BHs and Their Multiplicities},
author = {Ataru Tanikawa and Savannah Cary and Minori Shikauchi and Long Wang and Michiko S. Fujii},
journal= {arXiv preprint arXiv:2303.05743},
year = {2023}
}