FRICTIONAL HEATING AND NEUTRON-STAR THERMAL EVOLUTION

Differential rotation between the neutron star crust and a more rapidly rotating interior superfluid leads to frictional heating that effects the star's long-term thermal evolution and resulting surface emission. The frictional heating rate is determined by the mobility of the vortex lines that thread the rotating superfluid and pin to the inner crust lattice. If vortex pinning is relatively strong, a large velocity difference develops between the inner crust superfluid and the crust, leading to a high rate of heat generation by friction. Here we present the results of thermal evolution simulations based on two models of the vortex pinning forces that bracket a range of plausible pinning strengths. We include the effects of superfluidity, magnetic fields, and temperature gradients. As representative standard and accelerated neutrino emission processes taking place in the core, we consider the modified Urea process in normal baryonic matter, and the much faster quark Urea process. Comparison of our results with neutron star surface temperature data, including the recent temperature measurement of the Geminga pulsar, shows that stars with soft equations of state and modest frictional heating are in closest agreement with the data; stars with stronger frictional heating have temperatures inconsistent with the upper limit of PSR 1929+10. Stiffer stars undergoing standard cooling generally have temperatures lying above the Vela detection, a situation worsened by the inclusion of frictional heating. Stars undergoing accelerated cooling without frictional heating have temperatures that fall far below most temperature measurements; the Vela and Geminga detections being the most compelling examples. Only in stiff stars, which have thick crusts, can the inclusion of strong frictional heating raise the temperature at late stages in the evolution to a level consistent with the data. However, such a large amount of heating leads to a temperature at similar to 1000 yr in excess of the Crab upper limit. Suppression of accelerated neutrino emission processes, perhaps by superfluid pairing in the core, may yield acceptable cooling models.