Nonequilibrium Layer in the Crust of Neutron Stars and Nonequilibrium β-Processes in Astrophysics

Abstract

The formation of the chemical composition of neutron star envelopes, at densities 1010–1013 g cm–3, is considered. As hot matter is compressed in the process of collapse, which leads to the explosion of a core-collapse supernova, the stage of nuclear equilibrium with free neutrino escape, kinetic equilibrium in β-processes, and, as a result, the establishment of limited nuclear equilibrium with a fixed number of nuclei takes place. Cold matter is compressed at a fixed number of nuclei whose atomic weight initially does not change and subsequently decreases. A pycnonuclear reaction of the fusion of available nuclei and a decrease in their number begin at the end. The compression of cold matter is accompanied by an increase in the mass fraction of free neutrons. In this case, the chemical composition of the envelope differs significantly from the equilibrium one and contains a considerable store of nuclear energy. Nonequilibrium β-reactions proceed at densities exceeding the upper bound for the nonequilibrium layer density, which lead to heating, nuclear energy release, and the possible attainment of a state of complete thermodynamic equilibrium. The thermodynamics of nonequilibrium β-processes, which lead to the heating of matter as neutrinos escape freely, is considered.