Forming an “Impossible” 85 solar mass Black Hole

Abstract

At the end of its life, a very massive star is expected to collapse into a black hole. The masses of these black holes are pivotal for our understanding of the evolution and fate of these stars, as well as for galaxy evolution and the build-up of black hole masses through Cosmic time. The recent detection of an 85 solar mass black hole from the gravitational wave event GW 190521 appears to present a fundamental problem as to how such heavy black holes exist above the approximately 50 solar mass pair-instability limit where stars are expected to be blown to pieces with no remnant left. Here we show that for stellar models at reduced heavy element content, 90-100 solar mass stars can produce core masses sufficiently small to remain below the fundamental pair-instability limit, yet at the same time lose an amount of mass small enough to end up in an 85 solar mass black hole. A key point is that the amount of mass-loss scales with the host galaxy heavy element fraction, and not with the total amount of element enrichment that occurs naturally during the life of massive stars. Our study shows how our Universe is capable of producing heavy black holes, which are important seeds for the production of supermassive black holes that regulate the evolution of galaxies. Our evolutionary channel to the formation of an 85 solar mass black hole is of fundamental relevance for the manner in which metals are released in the outflows and explosions of the most massive stars, which is shown to be a strong function of Cosmic time.

Evolution of Wolf–Rayet stars as black hole progenitors

“…In order to have increased final masses which do not enter the PPI regime, stars should have small convective cores. Vink et al (2020) shows that by maintaining a small core with low convective core overshooting ( f ov = 0.01), and retaining a large H envelope, the effective PI gap can be much smaller than commonly anticipated as even massive BHs up to about 90 M , such as the one seen in the GW event GW190521, may be formed.…”

“…Our results highlight that the NL00 wind prescription leads to no stellar BHs formed above the second BH mass gap, unlike with SV20. Extremely low metallicity environments such as I Zw 18 (∼ 0.02 Z ) provide an insight into the early Universe and likely host some of the heaviest stellar mass black holes which may be detected as gravitational wave sources (Vink et al 2020). Calculations with the SV20 description show final masses of up to 140 M at 0.02 Z , hinting that there might be a channel for very massive He stars to form black holes above the second BH mass gap, though the most massive BHs may form from H-rich stars.…”

“…As such, Abbott et al (2020) suggested this extreme GW event to be the product of second generation BHs. Stellar evolution studies such as Vink et al (2020); Farrell et al (2021) have however hypothesised alternative solutions for creating first generation heavy BHs such as in GW 190521 by maintaining a low core mass and retaining the E-mail: erin.higgins@armagh.ac.uk large H envelope, which might even be possible for metallicities up to 0.1 Z (Vink et al 2020).…”