MECHANICS AND ENERGETICS OF VORTEX UNPINNING IN NEUTRON-STARS

A rotating neutron superfluid is threaded by quantized vortex lines which, in the inner crust of a neutron star, can pin to the nuclear lattice that coexists with the superfluid. In the nuclear pinning region of the stellar crust (stellar densities greater-than-or-similar-to 10(13) g cm-3) the lines pin to nuclei, while in the interstitial pinning region (densities less-than-or-similar-to 10(13) g cm-3) the lines pin between nuclei. Some of the peculiar timing features of pulsars may be due to the unpinning and movement of these vortex lines. The nature of vortex unpinning is determined by the relative importance of tension in the vortex line and the pinning forces acting upon it. If the vortex is relatively flexible, only one or a few bonds break simultaneously, whereas if it is relatively stiff, many pinning bonds break. For the pinning parameters of Epstein and Baym, we find that in the nuclear pinning region the vortex line is comparatively flexible, while for the interstitial pinning regions the vortex is stiff. For nuclear pinning energies weaker than those considered here, tension would be relatively more significant. In the interstitial pinning region, pinning energies are the order of 1 keV per site, and vortices can remain pinned for velocity differences upsilon-delta between the superfluid and the crust of upsilon-B approximately 10(5) cm s-1. For 3-upsilon-B/4 less-than-or-equal-to upsilon-delta < upsilon-B, pinned vortices may unpin by overcoming an energy activation barrier whose height varies as (1 - upsilon-delta/upsilon-B)3/2j* where j* is the characteristic number of pinning bonds involved in the unpinning event. As upsilon-delta becomes small, the activation energy diverges as upsilon-B/upsilon-delta.