Statistical approach for supernova matter

We formulate a statistical approach for description of nuclear composition and equation of state of stellar matter at subnuclear densities and temperature up to 20 MeV, which are expected during the collapse and explosion of massive stars. The model includes nuclear, electromagnetic and weak reactions between all constituents of matter, under condition of statistical equilibrium. We construct a realistic ensemble of nuclear species, and demonstrate the differences from the models employing a single-nucleus approximation for heavy nuclei. Thermodynamical characteristics, isentropic trajectories, EOS, and nuclear composition of supernova matter are calculated for different values of the lepton fraction. Special attention is paid to a possible in-medium reduction of the nuclear symmetry energy, as suggested by nuclear multifragmentation experiments. We demonstrate that this effect may lead to significant modifications of the nuclear composition and electron capture rates, which are important for stellar dynamics and nucleosynthesis.