  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.

Astrophysics;Nuclear Theory