  Nucleon effective masses are studied in the framework of the
Brueckner-Hartree-Fock many-body approach at finite temperature.
Self-consistent calculations using the Argonne $V_{18}$ interaction including
microscopic three-body forces are reported for varying temperature and proton
fraction up to several times the nuclear saturation density. Our calculations
are based on the exact treatment of the center-of-mass momentum instead of the
average-momentum approximation employed in previous works. We discuss in detail
the effects of the temperature together with those of the three-body forces,
the density, and the isospin asymmetry. We also provide an analytical fit of
the effective mass taking these dependencies into account. The temperature
effects on the cooling of neutron stars are briefly discussed based on the
results for betastable matter.
