Evolution of open clusters with or without black holes

TL;DR

This study uses N-body simulations to examine how lower natal kicks for black holes, due to fallback effects, influence their retention in star clusters and affect long-term cluster evolution.

Contribution

It demonstrates that lower natal kicks enable black hole retention, which accelerates mass segregation and influences cluster dissolution over time.

Findings

Lower natal kicks lead to retention of black holes in clusters.

Retained black holes accelerate mass segregation.

Cluster dissolution occurs faster with black hole retention.

Abstract

Binary black holes (BHs) can be formed dynamically in the centers of star clusters. The high natal kicks for stellar-mass BHs used in previous works made it hard to retain BHs in star clusters. Recent studies of massive star evolution and supernovae (SN) propose kick velocities that are lower due to the fallback of the SN ejecta. We study the impact of these updates by performing $N$-body simulations following instantaneous gas expulsion. For comparison, we simulate two additional model sets with the previous treatment of stars: one with high kicks and another with artificial removal of the kicks. Our model clusters initially consist of about one hundred thousand stars, formed with centrally-peaked efficiency. We find that the updated treatment of stars, due to the fallback-scaled lower natal kicks, allows clusters to retain SN remnants after violent relaxation. The mass contribution of the retained remnants does not exceed a few percent of the total bound cluster mass during the early evolution. For this reason, the first giga year of evolution is not affected significantly by this effect. Nevertheless, during the subsequent long-term evolution, the retained BHs accelerate mass segregation, leading to the faster dissolution of the clusters.