Magnetic fields within color superconducting neutron star cores

We discuss the Meissner effect for a color superconductor formed by cold de nse quark matter. Though color and ordinary electromagnetism are broken in a color superconductor, there is a linear combination of the photon and a g luon that remains massless. Consequently, a color superconducting region ma y be penetrated by an external magnetic field. We show that at most a small fraction of the magnetic field is expelled, and if the screening distance is the smallest length scale in the problem there is no expulsion at all. W e calculate the behavior of the magnetic field for a spherical geometry rel evant for compact stars. If a neutron star contains a quark matter core, th is core is a color superconductor. Our results demonstrate that such cores admit magnetic fields without restricting them to quantized flux tubes. Suc h magnetic fields within color superconducting neutron star cores are stabl e on time scales longer than the age of the universe, even if the spin peri od of the neutron star is changing. (C) 2000 Elsevier Science B.V. All righ ts reserved.