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

TL;DR

This paper investigates how triple star systems influence black hole merger properties, revealing a flatter mass ratio distribution than other formation channels, and discusses how different origins can be distinguished with gravitational wave data.

Contribution

It demonstrates that black hole mergers from triple systems produce a distinct, flatter mass ratio distribution compared to binaries and clusters, enhancing understanding of their population characteristics.

Findings

Triple systems yield a flatter mass ratio distribution than binaries or clusters.

Hierarchical mergers in clusters can produce asymmetric mass ratios with distinctive spins.

Different formation channels occupy distinct regions in total mass vs. mass ratio space.

Abstract

Observations have shown that the majority of massive stars, 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 the 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 $q\sim0.3$ 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 distinguished them from those produced from stellar triples. All three channels occupy distinct regions in total mass-mass ratio space, which may allow them to be disentangled as more BH mergers are detected by LIGO, Virgo, and KAGRA.