Superfluid friction and late-time thermal evolution of neutron stars

The recent temperature measurements of the two Older isolated neutron stars PSR 1929+10 and PSR 0950+08 (ages of 3 x 10(6) and 2 x 10(7) yr, respectiv ely) indicate that these objects are heated. A promising candidate heat sou rce is friction between the neutron star crust and the superfluid it is tho ught to contain. We study the effects of superfluid friction on the long-te rm thermal and rotational evolution of a neutron star. Differential rotatio n velocities between the superfluid and the crust (averaged over the inner crust moment of inertia) of <(omega)over bar> similar to 0.6 rad s(-1) for PSR 1929+10 and similar to 0.02 rad s(-1) for PSR 0950+08 would account for their observed temperatures. These differential velocities could be sustai ned by the pinning of superfluid vortices to the inner crust lattice with s trengths of similar to 1 MeV per nucleus. Pinned vortices can creep outward through thermal fluctuations or quantum tunneling. For thermally activated creep, the coupling between the superfluid and crust is highly sensitive t o temperature. If pinning maintains large differential rotation (similar to 30 rad s(-1)), a feedback instability could occur in stars younger than si milar to 10(5) yr causing oscillations of the temperature and spin-down rat e over a period of similar to 0.3t(age). For stars older than similar to 10 (6) yr, however, vortex creel! occurs through quantum tunneling and the cre ep velocity is too insensitive to temperature for a thermal-rotational inst ability to occur. These older stars could be heated through a steady proces s of superfluid friction.