Superfluid Friction and Late‐Time Thermal Evolution of Neutron Stars

Abstract

The recent temperature measurements of the two older isolated neutron stars PSR 1929{plus}10 and PSR 0950{plus}08 (ages of 3{times}10{sup 6} and 2{times}10{sup 7} yr, respectively) indicate that these objects are heated. A promising candidate heat source is friction between the neutron star crust and the superfluid it is thought to contain. We study the effects of superfluid friction on the long-term thermal and rotational evolution of a neutron star. Differential rotation velocities between the superfluid and the crust (averaged over the inner crust moment of inertia) of {bar {omega}}{approximately}0.6 rad s{sup {minus}1} for PSR 1929{plus}10 and {approximately}0.02 rad s{sup {minus}1} for PSR 0950{plus}08 would account for their observed temperatures. These differential velocities could be sustained by the pinning of superfluid vortices to the inner crust lattice with strengths of {approximately}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 to temperature. If pinning maintains large differential rotation ({approximately}30 rad s{sup {minus}1}), a feedback instability could occur in stars younger than {approximately}10{sup 5} yr causing oscillations of the temperature and spin-down rate over a period of {approximately}0.3t{sub age}. For stars older thanmore » {approximately}10{sup 6} yr, however, vortex creep occurs through quantum tunneling and the creep velocity is too insensitive to temperature for a thermal-rotational instability to occur. These older stars could be heated through a steady process of superfluid friction. {copyright} {ital {copyright} 1999.} {ital The American Astronomical Society}« less