  The thermal properties of asymmetric nuclear matter are investigated in a
relativistic mean- field approach. We start from free space NN-interactions and
derive in-medium self-energies by Dirac-Brueckner theory. By the DDRH procedure
we derive in a self-consistent approach density- dependent meson-baryon
vertices. At the mean-field level, we include isoscalar and isovector scalar
and vector interactions. The nuclear equation of state is investigated for a
large range of total baryon densities up to the neutron star regime, the full
range of asymmetries from symmetric nuclear matter to pure neutron matter, and
temperatures up to T~100 MeV. The isovector-scalar self-energies are found to
modify strongly the thermal properties of asymmetric nuclear matter. A striking
result is the change of phase transitions when isovector-scalar self-energies
are included.
