Correlation energy of nuclear matter and neutron star masses

We consider nuclear matter in the frames of the sigma model and find the role of correlation energy in the determination of the parameters of neutron stars. The response-function formalism is used for calculations within the Hartree-Fock approach and beyond. When electrons and muons are present in the neutron-rich matter, the maximal mass of the star is M*=1.64 (in the unit of the solar mass M·). The correlation energy becomes very important for the stars with M*∼0.7 divide 1.5M· and its effect is estimated as 0.3 divide 0.4M· extracted from the relevant values obtained in the frames of the Hartree-Fock approximation. On the whole, the nuclear equation of state is definitely 'softened'.