  In the framework of relativistic mean field theory, the relativistic neutrino
emissivity of the nucleonic and hyperonic direct Urca processes in the
degenerate baryon matter of neutron stars are studied. We investigate
particularly the influence of the isovector scalar interaction which is
considered by exchanging $\delta$ meson on the nucleonic and hyperonic direct
Urca processes. The results indicate that $\delta$ mesons lead to obvious
enhancement of the total neutrino emissivity, which must result in more rapid
cooling rate of neutron star matter.
