Rearrangement of the Fermi surface of dense neutron matter and the direct Urca cooling of neutron stars

It is proposed that a rearrangement of single-particle degrees of freedom m ay 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-is ospin collective mode which, under increasing density, leads to pion conden sation. Arguments and estimates based on fundamental relations of many-body theory show that one realization of this phenomenon could produce very rap id cooling of the star via a direct nucleon Urea process displaying a T-5 d ependence on temperature.