Collisions versus stellar winds in the runaway merger scenario: place your bets

TL;DR

This paper uses N-body simulations to explore how stellar winds and metallicity influence the formation of intermediate-mass black holes through runaway mergers in star clusters, with implications for gravitational wave detection.

Contribution

It introduces new recipes for stellar winds and supernovae in simulations, showing how metallicity affects the mass of merger remnants and the likelihood of IMBH formation.

Findings

At solar metallicity, merger remnants are up to ~30 solar masses.

At low metallicity, remnants can reach ~250 solar masses, consistent with IMBHs.

A significant fraction of remnants remain bound and may form binary systems.

Abstract

The runaway merger scenario is one of the most promising mechanisms to explain the formation of intermediate-mass black holes (IMBHs) in young dense star clusters (SCs). On the other hand, the massive stars that participate in the runaway merger lose mass by stellar winds. This effect is tremendously important, especially at high metallicity. We discuss N-body simulations of massive (~6x10^4 Msun) SCs, in which we added new recipes for stellar winds and supernova explosion at different metallicity. At solar metallicity, the mass of the final merger product spans from few solar masses up to ~30 Msun. At low metallicity (0.01-0.1 Zsun) the maximum remnant mass is ~250 Msun, in the range of IMBHs. A large fraction (~0.6) of the massive remnants are not ejected from the parent SC and acquire stellar or black hole companions. Finally, I discuss the importance of this result for gravitational wave detection.