Direct Urca processes in superdense cores of neutron stars

Abstract

We use the field theoretical model to perform relativistic calculations of neutrino energy losses caused by the direct Urca processes on nucleons in the degenerate baryon matter. By our analysis, in a free nucleon gas under beta equilibrium, the direct neutron decay is forbidden if the number density of neutrons exceeds the critical value $n_{n}^{c}=5.9\times 10^{31} cm^{-3}$. In superdense nuclear matter, $n>n_{0}$, the weak decay of neutrons is possible only due to strong interactions, caused by an exchange of isovector mesons. Mean field of isovector mesons in the medium creates a large energy gap between spectrums of protons and neutrons, which is required by kinematics of beta decay. Our expression for the neutrino energy losses, obtained in the mean field approximation, incorporates the effects of nucleon recoil, parity violation, weak magnetism, and pseudoscalar interaction. For numerical testing of our formula, we use a self-consistent relativistic model of the multicomponent baryon matter. The relativistic emissivity of the direct Urca reactions is found substantially larger than predicted in the non-relativistic approach. We found that, due to weak magnetism effects, relativistic emissivities increase by approximately 40-50%.