Inner crust of neutron stars: Unified description of structure and superconductivity

Abstract: Using the Skyrme model Sk$\chi$450 constrained by the chiral effective field theory and the ground-state energies of doubly-magic nuclei, we explore the macroscopic static energy spectrum of dense matter. Structure of the matter is idealized as 1-, 2- or 3-dimensional periodic Coulomb lattices (pasta phases) at average baryon density in the range of $0.005n_0-0.5n_0$ with $n_0\sim0.16$ fm$^{-3}$, corresponding to the inner crust of neutron stars. In the earlier work, the bulk and the surface nuclear properties in this scenario were described on the basis of different nuclear interactions. As a result, predictions for the inner crust structure based on those numerical results are not necessarily self-consistent. In this work, we solve this problem by describing the nuclear properties and proton-proton Cooper pairing in a unified manner, \emph{i.e.} based on the same nuclear interaction. We calculate the surface tension to the leading order (planar surface) and to the next-to-leading order (from nonzero principal curvature) at the Extended Thomas-Fermi level. Next, we develop and solve a system of equations within the compressible liquid drop model (LDM), which provides us with all necessary information about the macroscopic static energy spectrum of the nuclear structure. We find that the curvature corrections change the ground state in a relevant way. We also find that, typically, the energy differences between the different pasta phases are less than the thermal energy for temperatures $\sim10^8-10^9$ K, which implies that the real nuclear pasta phases are likely polymorphic. Finally, for 1-dimensional pasta phase we evaluate the superconducting coherence length of protons, the London penetration depth and the superconducting energy gap. Our results offer a preliminary insight into rich magnetic properties of the pasta phases.