Very Massive, Rapidly Spinning Binary Black Hole Progenitors through Chemically Homogeneous Evolution -- The Case of GW231123

Abstract

Among the over 200 gravitational wave detections reported so far, GW231123 is a remarkable event that not only holds the record for the most massive black hole merger, but also exhibits extreme spins. Its origin is actively debated. Proposed scenarios include dynamical formation involving a sequence of mergers, Population III stars, accretion in an AGN disk and also more exotic explanations including primordial black holes and cosmic strings, each facing different challenges. Recent work showed that the incoming black holes of GW231123 can be formed out of massive rapidly rotating collapsing helium stars. Here, we address the question how such very massive rapidly rotating helium stars can be formed in very close binary systems. For this we explore chemically homogeneous evolution (CHE) involving progenitors with masses above the pair-instability mass gap. We compute a grid of detailed massive binary models with the stellar evolution code MESA to follow the early evolution of binary progenitors and show that: (i) very massive ($M_i > 140\, M_\odot$) CHE binaries at low metallicity ($Z=10^{-5}$) naturally produce rapidly rotating progenitors with high masses and high spins matching the properties of the black holes in GW231123 and (ii) the maximum spin of the progenitors is bound by their critical rotation rate leading to a tight correlation between the dimensionless spin and mass, $a \propto M^{-0.9}$, in models that have no hydrogen left. We conclude that the CHE channel appears to be a viable and natural scenario to produce progenitors. We compare and discuss the differences with earlier studies and comment on the large uncertainties in the final fate and collapse.