HADRONIC PHASE-TRANSITIONS AT SUPRANUCLEAR DENSITIES AND THE DELAYED COLLAPSE OF NEWLY FORMED NEUTRON-STARS

We present numerical simulations of the neutrino-driven deleptonizatio n and cooling of newly formed, hot, lepton-rich neutron stars. In part icular, we are interested in the effects associated with the creation of additional hadronic states like hyperons and Delta-resonances at de nsities of a few times nuclear density. Due to the occurrence of these baryonic components besides neutrons and protons, the considered obje cts are nuclear matter stars rather than neutron stars. The hyperonic equation of state employed in this work yields stable, lepton-rich sta rs with a maximum (baryonic) mass which is higher by about Delta M(b) congruent to 0.17 M. compared with the stability limit of M(b)(max) co ngruent to 1.77 M. for deleptonized, cold matter. We find stars in the range 1.77 M. less than or similar to M(b) less than or similar to 1. 94 M. to be stabilized for an intermediate phase following their forma tion in the gravitational collapse of the stellar core. They become un stable against gravity after they have lost a significant fraction of their leptons, but long before they have cooled down. Typically, this stable period lasts for a few seconds up to about ten seconds. We do n ot see much of a stabilizing influence of thermal pressure, but therma l energy contributes to the gravitating mass and may bring the star hi gher above the stability limit in the deleptonized state, M(g)(max) ap proximate to 1.58 M.. Thus the additional mass that is supported by th ermal pressure of baryons turns out to be quite small, and the window of stabilized masses is increased by less than 0.1 M. to be about Delt a M(b) approximate to 0.25 M. for hot stars. We calculate the expected neutrino signals in the Kamiokande II and IMB detectors for our sampl e of protoneutron star models and compare with characteristic paramete rs of the neutrino observation in connection with SN 1987 A. Since our unstable stars approach the moment of dynamical collapse while they g radually become more and more compact, their continuously increasing g ravitational redshift and rising neutrino opacity suppress the observa ble neutrino signal, until the neutrino emission is finally chopped of f at the onset of collapse. We do not find an indication of a late out burst of neutrino radiation.