Type I Superconductivity in Neutron Stars

The magnetic structure of neutron vortices in the superfluid cores of neutron stars is determined assuming that the proton condensate forms a type I superconductor. It is shown that the entrainment currents induced by the neutron vortex circulation cause the proton superconductor to break into successive domains of normal and superconducting regions. The Gibbs free-energy is found in the case in which the normal domains form cylindrical tubes coaxial with the neutron vortex. The minimum of the energy functional corresponds to a tube radius $a\sim 0.1-0.5 ~b$, where $b$ is the outer radius of the neutron vortex. The magnetic field within the tube is of the order of $5 \times 10^{14}$ G.