Gravitational-wave constraints on the pair-instability mass gap and nuclear burning in massive stars

Abstract

Pair-instability should prevent the direct formation of black holes above about $50M_{\odot }$ creating a pair-instability mass gap. Yet gravitational-wave observations have detected black holes in this mass range. These systems can be explained with uncertainties in massive-star evolution, or hierarchical mergers in stellar clusters, which are expected to produce large spins with isotropic orientations. Here we present evidence for the pair-instability mass gap in the LIGO--Virgo--KAGRA fourth transient catalog, with a lower edge at $44.3_{-3.5}^{+5.9}{M}_{\odot }$. We also obtain a measurement of the ${}^{12}\mathrm{C}(\alpha ,\gamma ){}^{16}\mathrm{O}$ reaction rate, yielding an $S$-factor of $268_{-116}^{+195}\mathrm{keV}\mathrm{b}$, a parameter critical for modeling helium burning and stellar evolution. The data reveal two populations: a low-spin group with no black holes above the gap, and a high-spin, isotropic group that extends across the full mass range and occupies the gap, consistent with hierarchical mergers. These findings are consistent with pair-instability playing a role in shaping the black hole mass spectrum, point to a connection between gravitational wave astronomy and nuclear astrophysics, and highlight dense stellar clusters as key environments in the growth of black holes.

arXiv commentAccepted in Nature Astronomy

Cited by in corpus (1)

• Nuclear astrophysics