Is GW190521 the merger of black holes from the first stellar generations?

TL;DR

This paper explores whether GW190521's black hole merger could originate from first-generation stars, using stellar evolution models at low metallicity to explain the formation of massive black holes within the pair-instability mass gap.

Contribution

It presents new stellar evolution models at zero and low metallicity, showing conditions that allow formation of black holes in the mass gap consistent with GW190521.

Findings

Models with low metallicity favor higher black hole masses.

Black holes up to 70-75 solar masses can form from first-generation stars.

Uncertainties could increase the maximum black hole mass to ~85 solar masses.

Abstract

GW190521 challenges our understanding of the late-stage evolution of massive stars and the effects of the pair-instability in particular. We discuss the possibility that stars at low or zero metallicity could retain most of their hydrogen envelope until the pre-supernova stage, avoid the pulsational pair-instability regime and produce a black hole with a mass in the mass gap by fallback. We present a series of new stellar evolution models at zero and low metallicity computed with the Geneva and MESA stellar evolution codes and compare to existing grids of models. Models with a metallicity in the range 0-0.0004 have three properties which favour higher BH masses as compared to higher metallicity models. These are (i) lower mass-loss rates during the post-MS phase, (ii) a more compact star disfavouring binary interaction and (iii) possible H-He shell interactions which lower the CO core mass. We conclude that it is possible that GW190521 may be the merger of black holes produced directly by massive stars from the first stellar generations. Our models indicate BH masses up to 70-75 Msun. Uncertainties related to convective mixing, mass loss, H-He shell interactions and pair-instability pulsations may increase this limit to ~85 Msun.