On the Mass Ratio Distribution of Black Hole Mergers in Triple Systems

Observations have shown that the majority of massive stars, the progenitors of black holes (BHs), have on average more than one stellar companion. In triple systems, wide inner binaries can be driven to a merger by a third body due to long-term secular interactions, most notably by the eccentric Lidov-Kozai effect. In this study, we explore the properties of BH mergers in triple systems and compare their population properties to those of binaries produced in isolation and assembled in dense star clusters. Using the same stellar physics and identical assumptions for the initial populations of binaries and triples, we show that stellar triples yield a significantly flatter mass ratio distribution from q = 1 down to q similar to 0.3 than either binary stars or dense stellar clusters, similar to the population properties inferred from the most recent catalog of gravitational-wave events, though we do not claim that all the observed events can be accounted for with triples. While hierarchical mergers in clusters can also produce asymmetric mass ratios, the unique spins of such mergers can be used to distinguish them from those produced from stellar triples. All three channels occupy distinct regions in the total mass-mass ratio space, which may allow them to be disentangled as more BH mergers are detected by LIGO, Virgo, and KAGRA.

Automated summary

Observations have shown that most massive stars, which are the progenitors of black holes, usually have more than one stellar companion. In triple systems, the merging of wide inner binaries can be caused by a third body through long-term secular interactions, especially the eccentric Lidov-Kozai effect. This study explores the characteristics of black hole mergers in triple systems and compares them to the properties of binaries formed in isolation and in dense star clusters. The results demonstrate that stellar triples have a flatter mass ratio distribution compared to binary stars or dense stellar clusters, which is consistent with the properties of observed gravitational-wave events.