Collapse of Axion Stars
  Axion stars, gravitationally bound states of low-energy axion particles, have
a maximum mass allowed by gravitational stability. Weakly bound states
obtaining this maximum mass have sufficiently large radii such that they are
dilute, and as a result, they are well described by a leading-order expansion
of the axion potential. Heavier states are susceptible to gravitational
collapse. Inclusion of higher-order interactions, present in the full
potential, can give qualitatively different results in the analysis of
collapsing heavy states, as compared to the leading-order expansion. In this
work, we find that collapsing axion stars are stabilized by repulsive
interactions present in the full potential, providing evidence that such
objects do not form black holes. In the last moments of collapse, the binding
energy of the axion star grows rapidly, and we provide evidence that a large
amount of its energy is lost through rapid emission of relativistic axions.
