Rearrangement of the Fermi Surface of Dense Neutron Matter and Direct   Urca Cooling of Neutron Stars
  It is proposed that a rearrangement of single-particle degrees of freedom may occur in a portion of the quantum fluid interior of a neutron star. Such a rearrangement is associated with the pronounced softening of the spin-isospin collective mode which, under increasing density, leads to pion condensation. Arguments and estimates based on fundamental relations of many-body theory show that one realization of this phenomenon could produce very rapid cooling of the star via a direct nucelon Urca process displaying a $T^5$ dependence on temperature. 
D.N.Voskresensky, V.A.Khodel, M.V.Zverev and J.W.Clark