The neutrino emissivities resulting from direct Urca processes in neutron stars are calculated in a relativistic Dirac-Hartree approach in the presence of a magnetic field. In a quark or a hyperon matter environment, the emissivity due to the nucleon direct Urca process is suppressed relative to that from pure nuclear matter due to the appearance of new degrees of freedom. In all the cases studied, the magnetic field enhances emissivity compared to the field-free cases because of phase space modifications and a shift in the dynamical $\beta$-equilibrium condition.

• Received 9 April 1998

DOI:https://doi.org/10.1103/PhysRevD.58.121301