The dependence of black-hole formation in open clusters on the cluster formation process

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

^★ Corresponding author: jwzhou@mpifr-bonn.mpg.de

Received: 26 February 2025
Accepted: 15 October 2025

Abstract

We performed N-body simulations of both individual cluster evolution and subcluster coalescence, demonstrating that cluster evolution and its outcomes strongly depend on the cluster formation process through comparisons of different gas expulsion modes and formation channels. The evolution of star clusters is significantly shaped by the gas expulsion mode, with faster expulsion producing greater mass loss. A broader degeneracy exists among initial cluster mass, gas expulsion timescale, and formation channel (monolithic vs. coalescence), which manifests in both evolutionary pathways and black-hole production. In individual cluster simulations, slower gas expulsion enables progressively lower mass clusters to retain central black holes within the tidal radius. As the gas expulsion mode transitions from fast to moderate to slow, the fraction of high-velocity stars decreases. Variations in gas expulsion mode and formation channel ultimately influence the stellar velocity distribution (within the tidal radius) and, thus, the expansion speed, which governs both cluster mass loss and black-hole retention. Slowly expanding clusters are more likely to retain black holes and multiple systems, making them prime candidates for black-hole searches with Gaia. Our results highlight the crucial influence of early gas expulsion and cluster formation mechanisms on the dynamical evolution of star clusters and black-hole production. These factors should be carefully incorporated into the initial conditions of N-body simulations, which necessarily rely on input from the star formation community.