On the formation of exotic, massive, stellar-remnant black holes at solar and sub-solar metallicities through evolution of massive binaries

TL;DR

This paper explores how massive stellar-remnant black holes, exceeding 50 solar masses, can form at solar and globular cluster metallicities through binary evolution and accretion processes, challenging previous expectations.

Contribution

It demonstrates that massive black holes up to 80 solar masses can form at solar metallicity via accretion in binary systems, introducing the concept of black hole Thorne-Zytkow objects.

Findings

Black holes of 50-80 solar masses can form at solar metallicity with significant accretion.

At globular cluster metallicities, black holes can reach up to 100 solar masses.

Post-accretion black holes may have low or high spins, but this remains speculative.

Abstract

The recent inference of a 70M_sun black hole (BH) in the Galactic, detached binary LB-1 has sparked cross-disciplinary debate since a stellar remnant of such large mass is well above what can be expected from stellar-evolutionary theory, especially in an enriched environment like that of the Milky Way. This study focusses on the possibilities of formation of extraordinarily massive BHs at solar and globular cluster (GC)-like metallicities via evolution of massive stellar binaries. A population-synthesis approach is followed utilizing the recently updated BSE program. BHs in the mass range of 50M_sun-80M_sun could be formed at the solar metallicity only if a large fraction, >70%, of matter is allowed to accrete onto a low-mass BH, in a BH-star merger product (a "black hole Thorne-Zytkow object"; BH-TZO). Their counterparts at GC-like metallicities can reach 100 M_sun. Although post-accretion BHs can, generally, be expected to be of high spin parameter, they can potentially be of low spin in the case of a BH-TZO. This spin aspect remains speculative in this work and deserves detailed hydrodynamic studies.