Starquake-induced magnetic field and torque evolution in neutron stars

The persistent increases in spin-down rate (offsets) seen to accompany glit ches in the Crab and other pulsars suggest increases in the spin-down torqu e. We interpret these offsets as due to starquakes occurring as the star sp ins down and the rigid crust becomes less oblate. We study the evolution of strain in the crust, the initiation of starquakes, and possible consequenc es for magnetic held and torque evolution. Crust cracking occurs as equator ial material shears under the compressive forces arising from the star's de creasing circumference and as matter moves to higher latitudes along a faul t inclined to the equator. A starquake is most likely to originate near one of the two points on the rotational equator farthest from the magnetic pol es. The material breaks along a fault approximately aligned with the magnet ic poles. We suggest that the observed offsets come about when a starquake perturbs the star's mass distribution, producing a misalignment of the angu lar momentum and spin axes. Subsequently, damped precession to a new rotati onal state increases the angle a between the rotation and magnetic axes. Th e resulting increase in external torque appears as a permanent increase in the spin-down rate. Repeated starquakes would continue to increase a, makin g the pulsar more of an orthogonal rotator.