Flux Expulsion and Field Evolution in Neutron Stars

Models for the evolution of magnetic fields of neutron stars are constructed,assuming the field is embedded in the proton superconducting core of the star.The rate of expulsion of the magnetic flux out of the core, or equivalently thevelocity of outward motion of flux-carrying proton-vortices is determined froma solution of the Magnus equation of motion for these vortices. A force due tothe pinning interaction between the proton-vortices and the neutron-superfluidvortices is also taken into account in addition to the other more conventionalforces acting on the proton-vortices. Alternative models for the fieldevolution are considered based on the different possibilities discussed for theeffective values of the various forces. The coupled spin and magnetic evolutionof single pulsars as well as those processed in low-mass binary systems arecomputed, for each of the models. The predicted lifetimes of active pulsars,field strengths of the very old neutron stars, and distribution of the magneticfields versus orbital periods in low-mass binary pulsars are used to test theadopted field decay models. Contrary to the earlier claims, the buoyancy isargued to be the dominant driving cause of the flux expulsion, for the singleas well as the binary neutron stars. However, the pinning is also found to playa crucial role which is necessary to account for the observed low field binaryand millisecond pulsars.Comment: 23 pages, + 7 figures, accepted for publication in Ap