Flux expulsion and field evolution in neutron stars

Models for the evolution of magnetic fields of neutron stars are constructe d, assuming the held is embedded in the proton superconducting core of the star. The rate of expulsion of the magnetic flux out of the core or, equiva lently, the velocity of outward motion of flux-carrying proton vortices is determined from a solution of the Magnus equation of motion for these vorti ces. A force due to the pinning interaction between the proton vortices and the neutron-superfluid vortices, in addition to the other more conventiona l forces acting on the proton vortices, is also taken into account. Alterna tive models for the held evolution are considered based on the different po ssibilities discussed for the effective values of the various forces. The c oupled spin and magnetic evolution of single pulsars as well as those proce ssed in low-mass binary systems are computed for each of the models. The pr edicted lifetimes of active pulsars, the field strengths of the very old ne utron stars, and the distribution of the magnetic fields versus orbital per iods in low-mass binary pulsars are used to test the adopted field decay mo dels. Contrary to the earlier claims, buoyancy is argued to be the dominant driving cause of flux expulsion for single as well as binary neutron stars . However, the pinning is also found to play a crucial role that is necessa ry to account for the observed low field binary and millisecond pulsars.