Location and symmetry of superconductivity in neutron stars

Abstract: Earlier, it was a standard assumption that the entire core of neutron stars is superconducting. However, the matter contents in the inner core has been unknown even qualitatively, because the density of matter in that region is expected to be higher than the nuclear saturation density 0.16 $\mathrm{fm}^{-3}$. As a consequence, no reliable model exists that would describe the neutron star matter in the inner core of neutron stars. Thus, a possibility of presence of normal, nonsuperconducting, plasma in the inner core cannot be excluded as of today. This point is supported by the numerical calculations performed in [1]. The calculations are based on the equation of state and the proton Cooper pairing gap energy derived from the chiral effective field theory. The numerical results show that the superconducting gap goes to zero beyond the depth about 1 km below the crust-core boundary. Given that the stellar radius is of the order of 12 km, therefore the superconducting proton matter is expected to exist only in a thin layer at the tip of the outer core. Recently it has been realized that the symmetry of superconductor is anisotropic in the lasagna region of the pasta phases located at the bottom of the crust. However the question of whether this symmetry is continuous or discreet was unsolved. The numerical calculations performed in [1] have shown that the tunneling rate between the adjacent slabs in the entire range of the corresponding densities is negligibly small. Thus, a discreet model is necessary for the description of the lasagna region. Uncertainties and future directions of the research are discussed.