Filling the black hole mass gap: Avoiding pair instability in massive stars through addition of nonnuclear energy
Joshua Ziegler, Katherine Freese
Abstract
In standard stellar evolution, stars with masses ranging from approximately $150\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ to $240\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ are expected to evolve to a pair-instability supernova with no black hole (BH) remnant. This evolutionary behavior leads to a predicted gap in the black hole mass function from approximately $50\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ to $140\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$. Yet the LIGO and Virgo collaborations [Phys. Rev. Lett. 125, 101102 (2020)] recently discovered black holes with masses of $66\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ and $85\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ in the gravitational-wave event GW190521. We propose a new method to populate the BH mass gap. If an energy source is added throughout the star in addition to nuclear fusion, it is possible for the altered evolution to avoid the complete destruction of a pair-instability supernova, and instead a BH remnant is left behind. An example of an extra energy source is dark matter annihilation within the star, but our results hold more generally. We show this phenomenon by exploring the effect of adding an energy source independent of temperature and density to a $180\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ star, using the mesa one-dimensional stellar evolution software. $\text{If}\ensuremath{\sim}50%$ of the star's energy is due to this new source, the star is capable of avoiding the pair instability entirely, reaching a presupernova mass of $119\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$ before evolving into a BH remnant in the mass gap.