Critical Phenomena Associated with Boson Stars

We present a brief synopsis of related work (gr-qc/0007039), describing a study of black hole threshold phenomena for a self-gravitating, massive complex scalar field in spherical symmetry. We construct Type I critical solutions dynamically by tuning a one-parameter family of initial data consisting of a boson star and a massless real scalar field, and numerically evolving this data. The resulting critical solutions appear to correspond to boson stars on the unstable branch, as we show via comparisons between our simulations and perturbation theory. For low-mass critical solutions, we find small ``halos'' of matter in the tails of the solutions, and these distort the profiles which otherwise agree with unstable boson stars. These halos seem to be artifacts of the collisions between the original boson stars and the massless fields, and do not appear to belong to the true critical solutions. From this study, it appears that unstable boson stars are unstable to dispersal (``explosion'') in addition to black hole formation. Given the similarities in macroscopic stability between boson stars and neutron stars, we suggest that similar phenomena could occur in models of neutron stars.