Merging binary black holes formed through double-core evolution

TL;DR

This paper explores an alternative formation channel for merging binary black holes via double helium star evolution, incorporating detailed stellar and binary physics, and compares the results with observed gravitational wave events.

Contribution

It introduces detailed models of double-core evolution including rotation, mass loss, and tidal effects, providing new insights into black hole spin and mass distributions.

Findings

Tidal interactions influence helium star evolution at orbital periods under 1 day.

Higher metallicity environments affect black hole spins through angular momentum transport.

Double-core evolution can produce a range of black hole spins, not always fast.

Abstract

To date, various formation channels of merging events have been heavily explored with the detection of nearly 100 double black hole (BH) merger events reported by the LIGO-Virgo-KAGRA (LVK) Collaboration. We here systematically investigate an alternative formation scenario, i.e., binary BHs (BBHs) formed through double helium stars (hereafter double-core evolution channel). In this scenario, the two helium stars (He-rich stars) could be the outcome of the classical isolated binary evolution scenario involving with and without common-envelope phase (i.e., CE channel and stable mass transfer channel), or alternatively of massive close binaries evolving chemically homogeneously (i.e., CHE channel). We perform detailed stellar structure and binary evolution calculations that take into account internal differential rotation and mass loss of He-rich stars, as well as tidal interactions in binaries. For double He-rich stars with equal masses in binaries, we find that tides start to be at work on the Zero Age Helium Main Sequence (ZAHeMS: the time when a He-rich star starts to burn helium in the core, which is analogous to ZAMS for core hydrogen burning) for initial orbital periods not longer than 1.0 day, depending on the initial metallicities. Besides the stellar mass loss rate and tidal interactions in binaries, we find that the role of the angular momentum transport efficiency in determining the resulting BH spins, becomes stronger when considering BH progenitors originated from a higher metal-metallicity environment. We highlight that double-core evolution scenario does not always produce fast-spinning BBHs and compare the properties of the BBHs reported from the LVK with our modeling.