How viscous is a superfluid neutron star core?

Abstract

We discuss the effects of superfluidity on the shear viscosity in a neutron star core. Our study combines existing theoretical results for the viscosity coefficients with data for the various superfluid energy gaps into a consistent description. In particular, we provide a simple model for the electron viscosity which is relevant both when the protons form a normal fluid and when they become superconducting. This model explains in a clear way why proton superconductivity leads to a significant strengthening of the shear viscosity. We present our results in a form which permits the use of data for any given modern equation of state (our final formulas are explicitly dependent on the proton fraction). We discuss a simple description of the relevant superfluid pairing gaps, and construct a number of models (spanning the range of current uncertainty) which are then used to discuss the superfluid suppression of shear viscosity. We conclude by a discussion of a number of challenges that must be met if we are to make further progress in this area of research.