Evolution of Protoneutron Stars

We study the thermal and chemical evolution during the Kelvin-Helmholtz phase of the birth of a neutron star, employing neutrino opacities that are consistently calculated with the underlying equation of state (EOS). Expressions for the diffusion coefficients appropriate for general relativistic neutrino transport in the equilibrium diffusion approximation are derived. The diffusion coefficients are evaluated using a field-theoretical finite temperature EOS that includes the possible presence of hyperons. The variation of the diffusion coefficients is studied as a function of EOS and compositional parameters. We present results from numerical simulations of protoneutron star cooling for internal stellar properties as well as emitted neutrino energies and luminosities. We discuss the influence of the initial stellar model, the total mass, the underlying EOS, and the addition of hyperons on the evolution of the protoneutron star and upon the expected signal in terrestrial detectors. We find that the differences in predicted luminosities and emitted neutrino energies do not depend much upon the details of the initial models or the underlying high-density EOS for early times (t < 10 s), provided that opacities are calculated consistently with the EOS. The same holds true for models which allow for the presence of hyperons, except when the initial mass is significantly larger than the maximum mass for cold, catalyzed matter. For times larger than about 10 seconds, and prior to the occurrence of neutrino transparency, the neutrino luminosities decay exponentially with a time constant that is sensitive to the high-density properties of matter. We also find the average emitted neutrino energy increases during the first 5 seconds of evolution, and then decreases nearly linearly with time. In general, increasing the protoneutron star mass increases the average energy and the luminosity of neutrinos, as well as the overall evolutionary time scale. The influence of hyperons or variations in the dense matter EOS is increasingly important at later times. Metastable stars, those with hyperons which are unstable to collapse upon deleptonization, have relatively long evolution times, which increase the nearer the mass is to the maximum mass supported by a cold, deleptonized star. Subject headings: protoneutron star, neutrinos, supernova, stellar evolution