DELAYED COLLAPSE OF HOT NEUTRON-STARS TO BLACK-HOLES VIA HADRONIC PHASE-TRANSITIONS

We present numerical simulations of the delayed collapse of a hot nasc ent neutron star to a black hole. Using a recently developed, singular ity-avoiding dynamical code we can follow the collapse to completion a nd can study the late-time effects. We employ a hyperonic equation of state which is softer for deleptonized matter than for lepton-rich mat ter. For this equation of state the maximum mass of stable neutron sta rs therefore decreases as the protoneutron star loses lepton number by emission of electron neutrinos during the first seconds after its for mation in the core bounce in a supernova. Protoneutron stars with mass es within a critical window are therefore stable initially but later i nevitably collapse to a black hole. We study the last stages before su ch 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 neut ron 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 ap proaches the event horizon and the gravitational redshift becomes extr emely strong. Because muon and tau neutrinos as well as electron antin eutrinos decouple from deeper, hotter layers than electron neutrinos, they are usually emitted with higher mean energies. During the last mi llisecond before the neutron star goes into the black hole, however, t he gravitational redshift is so strong that the usual order of mean ne utrino energies and fluxes is inverted.