Gapless superfluidity in neutron stars: Thermal properties

Abstract

The interior of mature neutron stars is expected to contain superfluid neutrons and superconducting protons. The influence of temperature and currents on superfluid properties is studied within the self-consistent timedependent nuclear energy-density functional theory. We find that this theory predicts the existence of a regime in which nucleons are superfluid (the order parameter remains finite) even though the energy spectrum of quasiparticle excitations exhibits no gap. We show that the disappearance of the gap leads to a specific heat that is not exponentially suppressed at low temperatures as in the Bardeen-Cooper-Schrieffer regime but can be comparable to that in the normal phase. Introducing some dimensionless effective superfluid velocity, we show that the behavior of the specific heat is essentially universal and we derive general approximate analytical formulas for applications to neutron-star cooling simulations.