Vortex Unpinning in Precessing Neutron Stars

The neutron vortices thought to exist in the inner crust of a neutron star interact with nuclei and are expected to pin to the nuclear lattice. Evidence for long-period precession in pulsars, however, requires that pinning be negligible. We estimate the strength of vortex pinning and show that hydrodynamic forces present in a precessing star are likely sufficient to unpin all of the vortices of the inner crust. In the absence of precession, however, vortices could pin to the lattice with sufficient strength to explain the giant glitches observed in many radio pulsars.