The Pair-Instability Mass Gap for Black Holes

TL;DR

This paper investigates the theoretical boundaries of the black hole mass gap caused by pair instability, considering various factors like nuclear reactions, binary evolution, rotation, and accretion, to reconcile theory with recent LIGO observations.

Contribution

The study refines the estimates of the black hole mass gap boundaries by analyzing multiple astrophysical factors and their uncertainties, updating previous theoretical predictions.

Findings

Nuclear reaction rate uncertainties allow Mlo up to 64 solar masses.

Rotation can increase Mlo to about 70 solar masses.

Hyper-Eddington accretion and binary evolution could further expand the mass gap.

Abstract

Stellar evolution theory predicts a "gap" in the black hole birth function caused by the pair instability. Presupernova stars that have a core mass below some limiting value, Mlo, after all pulsational activity is finished, collapse to black holes, whereas more massive ones, up to some limiting value, Mhi, explode, promptly and completely, as pair-instability supernovae. Previous work has suggested Mlo is approximately 50 solar masses and Mhi is approximately 130 solar masses. These calculations have been challenged by recent LIGO observations that show many black holes merging with individual masses, Mlo is least some 65 solar masses. Here we explore four factors affecting the theoretical estimates for the boundaries of this mass gap: nuclear reaction rates, evolution in detached binaries, rotation, and hyper-Eddington accretion after black hole birth. Current uncertainties in reaction rates by themselves allow Mlo to rise to 64 solar masses and Mhi as large as 161 solar masses. Rapid rotation could further increase Mlo to about 70 solar masses, depending on the treatment of magnetic torques. Evolution in detached binaries and super-Eddington accretion can, with great uncertainty, increase Mlo still further. Dimensionless Kerr parameters close to unity are allowed for the more massive black holes produced in close binaries, though they are generally smaller.