Delayed collapse of hot neutron stars to black holes via hadronic phase transitions

We present numerical simulations of the delayed collapse of a hot nascent neutron star to a black hole. Using a recently developed, singularity-avoiding dynamical code we can follow the collapse to completion and can study the late-time effects. We employ a hyperonic equation of state which is softer for deleptonized matter than for lepton-rich matter. For this equation of state the maximum mass of stable neutron stars therefore decreases as the protoneutron star loses lepton number by emission of electron neutrinos during the first seconds after its formation in the core bounce in a supernova. Protoneutron stars with masses within a critical window are therefore stable initially but later inevitably collapse to a black hole. We study the last stages before such a collapse, as well as the final, dynamical implosion, tracking the evolution of the star until its surface reaches the event horizon. In particular, we determine the characteristics of the neutrino emission during this delayed collapse of the protoneutron star. Since hot neutron star matter is opaque to neutrinos, we find that there is no late increase or final, powerful outburst of the neutrino emission. Instead, the fluxes gradually decrease as more and more matter in the star approaches the event horizon and the gravitational redshift becomes extremely strong. Because muon and tau neutrinos as well as electron antineutrinos decouple from deeper, hotter layers than electron neutrinos, they are usually emitted with higher mean energies. During the last millisecond before the neutron star goes into the black hole, however, the gravitational redshift is so strong that the usual order of mean neutrino energies and fluxes is inverted.