  Thermodynamical properties of nuclear matter are studied in the framework of
an effective many-body field theory at finite temperature, considering the
Sommerfeld approximation. We perform the calculations by using the nonlinear
Boguta and Bodmer model, extended by the inclusion of the fundamental baryon
octet and leptonic degrees of freedom. Trapped neutrinos are included in order
to describe protoneutron star properties through the integration of the
Tolman-Oppenheimer-Volkoff equations, from which we obtain, beyond the standard
plots for the mass and radius of the protoneutron stars as functions of central
density, new plots of these quantities as functions of temperature. Our
predictions include the determination of an absolute value for the limiting
protoneutron star mass; new aspects on nuclear matter phase transition via the
behaviour of the specific heat and, through the inclusion of quark degrees of
freedom, the properties of a hadron-quark phase transition and of the hybrid
protoneutron stars.
