ACCRETION-INDUCED MAGNETIC-FIELD DECAY AND POLARIZED PROTONS IN THE NEUTRON-STAR CORE

We consider polarized protons in the core of a neutron star as the source of the magnetic field. The effective proton magnetic moment is density-dependent and changes sign at some density n(r) and so does the magnetization. Neutron stars with the central density close to n(r) have dipole magnetic field of the order of 10(12) G. For heavier stars the field decreases fast with the mass, goes through zero and then again increases, albeit in the opposite direction. The abrupt change of the magnetic field occurs on a mass scale of 0.1 solar mass. This model accounts for recent evidence that decay of magnetic field occurs only for neutron stars which accreted matter in their evolution. Conditions are discussed for the polarized proton phase to form the ground state.