Precession of isolated neutron stars. I. Effects of imperfect pinning

We consider the precession of isolated neutron stars in which superfluid is not pinned to the stellar crust perfectly. In the case of perfect pinning, Shaham showed that there are no slowly oscillatory, long-lived modes. When the assumption of perfect pinning is relaxed, new modes are found that can be long lived but are expected to be damped rather than oscillatory, unles s the drag force on moving superfluid vortex lines has a substantial compon ent perpendicular to the direction of relative motion. The response of a ne utron star to external torques, such as the spin-down torque, is also treat ed. We find that when computing the response of a star to perturbations, as suming perfect coupling of superfluid to normal matter from the start can m iss some effects.