Hypercritical accretion during common envelopes in triples leading to binary black holes in the pair-instability-supernova mass gap

TL;DR

This paper proposes a new stellar evolution model involving hypercritical accretion in triple systems' common envelopes, explaining the formation of binary black holes within the pair-instability supernova mass gap observed by gravitational waves.

Contribution

It introduces a novel accretion mechanism during common envelope phases in triples, enabling formation of massive binary black holes in the PISN mass gap, supported by population synthesis simulations.

Findings

Produced BBH masses consistent with GW190521

Identified Mk34 as a potential progenitor

Discussed spin distributions of observed mergers

Abstract

Hydrodynamic studies of stellar-mass compact objects (COs) in a common envelope (CE)have shown that the accretion rate onto the CO is a few orders of magnitude below the Bondi-Hoyle-Lyttleton (BHL) estimate. This is several orders of magnitude above the Eddington limit and above the limit for neutrino-cooled accretion (i.e., hypercritical accretion, or HCA). Considering that a binary system inside the CE of a third star accretes material at nearly the same rate as a single object of the same total mass, we propose stellar-evolution channels which form binary black hole (BBH) systems with its component masses within the pair-instability supernova (PISN) mass gap. Our model is based on HCA onto the BBH system engulfed into the CE of a massive tertiary star. Furthermore, we propose a mass transfer mode which allows to store mass lost by the binary onto a third star. Through the use of population synthesis simulations for the evolution of BBHs and standard binary-evolution principles for the interaction with a tertiary star, we are able to produce BBHs masses consistent with those estimated for GW190521. We also discuss the massive binary system Mk34 as a possible progenitor of BBHs in the PISN gap, as well as the spin distribution of the observed mergers in the gravitational-wave catalog.