Intermediate-mass Black Holes from High Massive-star Binary Fractions in Young Star Clusters

Abstract

Black holes formed in dense star clusters, where dynamical interactions are frequent, may have fundamentally different properties than those formed through isolated stellar evolution. Theoretical models for single star evolution predict a gap in the black hole mass spectrum from roughly $40-120M_{\odot }$ caused by (pulsational) pair-instability supernovae. Motivated by the recent LIGO/Virgo event GW190521, we investigate whether black holes with masses within or in excess of this "upper-mass gap" can be formed dynamically in young star clusters through strong interactions of massive stars in binaries. We perform a set of $N$-body simulations using the CMC cluster-dynamics code to study the effects of the high-mass binary fraction on the formation and collision histories of the most massive stars and their remnants. We find that typical young star clusters with low metallicities and high binary fractions in massive stars can form several black holes in the upper-mass gap and often form at least one intermediate-mass black hole. These results provide strong evidence that dynamical interactions in young star clusters naturally lead to the formation of more massive black hole remnants.

10 pages, 2 figures, 1 table. Accepted for publication in ApJ Letters. Comments welcome

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• Stellar-mass black holes in young massive and open stellar clusters V: comparisons with LIGO-Virgo merger rate densities
• Globular cluster formation histories, masses and radii inferred from gravitational waves
• Intermediate-mass Black Holes on the Run from Young Star Clusters
• Dynamics of Intermediate-Mass Black Holes Wandering in the Milky Way Galaxy Using the Illustris TNG50 Simulation
• An elusive dark central mass in the globular cluster M4
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• Detection of LIGO-Virgo binary black holes in the pair-instability mass gap
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• Massive black hole formation in dense stellar environments: Enhanced X-ray detection rates in high velocity dispersion nuclear star clusters
• Using Binaries in Globular Clusters to Catch Sight of Intermediate-Mass Black Holes