Physical properties of hot, dense matter: The bulk equilibrium approximation

Starting from a Skyrme effective nucleon-nucleon interaction, we calculate the properties of hot matter at densities up to a little beyond that of nuclear matter. We neglect effects associated with finite nuclear size, such as surface and Coulomb energies, but include all effects associated with the bulk properties of the neutron-proton matter over a range of proton concentrations. The region of coexistence of two distinct nuclear phases is mapped out, as a function of the average proton fraction of the two-phase mixture. We present the neutron and proton chemical potentials, and the densities and proton fractions of the two coexisting phases, as functions of density and temperature. The density dependence of the nuclear contribution to the pressure is given along isotherms for a range of proton fractions. Including electron, positron and photon contributions to the entropy, we calculate the density dependence of the pressure along adiabats, and discuss the distribution of the entropy among the components. Some information is also given about hot matter in β-equilibrium. Comparison is made with earlier work.