Shear viscosity of the outer crust of neutron stars: Ion contributions

The shear viscosity of the crust might have a damping effect on the amplitude of r-modes of rotating neutron stars. This damping has implications for the emission of gravitational waves. We calculate the contribution to the shear viscosity coming from the ions using both semianalytical methods that consider binary collisions and molecular dynamics simulations. We compare these results with the contribution coming from electrons. We study how the shear viscosity depends on density for conditions of interest in neutron star envelopes and outer crusts. In the low-density limit, we find good agreement between results of our molecular dynamics simulations and those of classical semianalytic calculations.