Binary black hole mergers from merged stars in the Galactic field

TL;DR

This paper proposes a new formation channel for binary black hole mergers in the galactic field via post-merger binaries from stellar triples, affecting the merger rate and mass ratio distribution.

Contribution

It introduces a novel pathway for black hole mergers involving stellar post-merger binaries from triples, with population synthesis estimates and implications for gravitational wave observations.

Findings

Post-merger binaries from triples can produce black hole mergers in the galactic field.

Estimated merger rate contribution is 0.3-25.2 Gpc^{-3} yr^{-1}.

Low mass ratio mergers dominate the rate and align better with GW data.

Abstract

The majority of massive stars are found in close binaries which: (i) are prone to merge and (ii) are accompanied by another distant tertiary star (triples). Here, we study the evolution of the stellar post-merger binaries composed of the merger product and the tertiary companion. We find that post-merger binaries originating from compact stellar triples with outer semi-major axes $a_{\rm out,init}\lesssim10^1\,-\,10^2\,\rm AU$ provide a new way to form binary black hole mergers in the galactic field. By means of a population synthesis, we estimate their contribution to the total black hole merger rate to be $\mathcal{R}(z=0)=0.3\,-\,25.2\,\rm Gpc^{-3}\,yr^{-1}$. Merging binary black holes that form from stellar post-merger binaries have exceptionally low mass ratios. We identify a critical mass ratio $q\simeq0.5$ below which they dominate the total black hole merger rate in the field. We show that after including their additional contribution, the mass ratio distribution of binary black hole mergers in the galactic field scenario is in better agreement with that inferred from gravitational wave detections.