Direct URCA process in neutron stars with strong magnetic fields

We calculate the emissivity for the direct URCA process in strongly magnetized, degenerate matter in neutron stars, under $\beta $-equilibrium. We show that, if magnetic field is large enough for protons and electrons to be confined to the ground Landau levels, the field-free threshold condition on proton concentration no longer holds, and direct URCA reactions are open for an arbitrary proton concentration. For densities around the saturation point and a magnetic field larger than $B\simeq 7\times 10^{17}G$, this process leads to fast neutron star cooling.