Chapter II Superfluidity in Neutron Star Matter and Symmetric Nuclear Matter

Nucleon superfluids which are realized in neutron star interior and symmetric nuclear matter are studied with use of realistic nuclear forces, in the density domain from the subnuclear region to about 3ρ_0 (ρ_0 being the nuclear density). It is shown that characteristic aspects of nuclear forces manifest themselves in the appearance of several kinds of nucleon superfluids, which strongly depends on the density ρ. In this chapter emphasis is put on the pairing correlations where strong noncentral (tensor and spin-orbit) forces play important roles. A theoretical framework applicable to the nonzero angular-momentum pairing including the coupling due to tensor force is given by extending the usual BCS-Bogoliubov theory for the ^1S_0 pairing (the zero angular-momentum one). This formulation has been applied to the ^3P_2+^3F_2 pairing in neutron matter (the dominant component of neutron stars) and the ^3S_1+^3D_1 pairing in symmetric nuclear matter. In the former case, although spin-orbit force mainly contributes to the ^3P_2 attraction, the tensor coupling with the ^3F_2 component assists to realize the ^3P_2 superfluid. In the latter case, the tensor coupling to the ^3D_1 component plays a vital role to realize the ^3S_1 superfluid with a large energy gap. Results of the energy gaps calculated for such nonzero angular-momentum pairings, as well as those for the ^1S_0 pairing, are shown. We have found the realization of the following nucleon superfluids; the neutron ^3P_2 superfluid and the proton ^1S_0 one in the fluid core of neutron stars at ρ≃(0.7∼3)ρ_0, the neutron ^1S_0 superfluid in the inner crust of neutron stars at ρ≃(10^−3∼0.5)ρ_0, and the ^3S_1 superfluid in symmetric nuclear matter at a wide range of ρ including ρ_0, contrary to the ^1S_0 one realized at ρ≲ρ_0/2. The properties of these superfluids and their implications are also discussed.