Critical Phenomena in Gravitational Collapses of Neutron Star Systems

We study critical phenomena in gravitational collapses of stellar objects based on numerical solutions of the Einstein equations. We discovered that stellar objects with large kinetic energy described by an equation of state (EOS) commonly used in describing neutron star matter may undergo critical collapses. To the best of our knowledge, this is the first study showing that critical collapses can occur without postulating an exotic EOS. We further showed that the critical collapses can occur due to an adiabatic change of the EOS, without fine tuning of the initial conditions. This opens the possibility that a neutron-star-like compact object with substantial kinetic energy, e.g. one newly formed in a supernova or in a binary coalescence, may undergo a critical collapse in processes which change the EOS, such as cooling. We investigated the properties of the critical solutions of this system. We found, surprisingly, that there are branches of critical solutions, and a complicated phase diagram not been seen in other critical collapse studies. The numerical simulations were carried out based on the full set of the Einstein equations coupled to the general relativistic hydrodynamic equations. The code we