Filling the Black Hole Mass Gap: Avoiding Pair Instability in Massive Stars through Addition of Non-Nuclear Energy

TL;DR

This paper proposes that adding an external energy source to massive stars can prevent pair instability supernovae, allowing the formation of black holes within the predicted mass gap, thus explaining recent gravitational wave observations.

Contribution

It introduces a novel stellar evolution mechanism involving external energy sources to avoid pair instability, enabling black hole formation in the mass gap.

Findings

Adding ~50% external energy prevents pair instability in a 180 M_sun star.

The modified evolution results in a black hole remnant of approximately 120 M_sun.

This mechanism can explain the existence of massive black holes observed by LIGO/Virgo.

Abstract

In standard stellar evolution, stars with masses ranging from approximately $150$ to $240 M_\odot$ 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 $50$ to $140 M_\odot$. Yet the LIGO and Virgo Collaborations[1] recently discovered black holes of masses $66 M_\odot$ and $85 M_\odot$ 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 $180 M_\odot$ star, using the MESA one-dimensional stellar evolution software. If $\sim50$\% of the star's energy is due to this new source, the star is capable of avoiding the pair instability entirely and evolving towards a core-collapse supernova and ultimately a BH remnant with mass $\sim 120 M_\odot$.