The spin evolution of a strongly magnetic star due to interaction with an accretion disc

The spin behaviour of a strongly magnetic star accreting from an internally disrupted disc is considered. The torques are calculated due to the interaction of the star with the disc and the accretion flow, together with the modified structure of the disc. The torques depend on the spin period of the star, and a stable equilibrium period can be found at which the disc and accretion torques cancel. The cases of a neutron star in an X‐ray binary pulsar and a white dwarf in an intermediate polar are analysed. The spin period evolution rates agree with the observed values within a plausible range of system parameters. The neutron star disc shows significant local thickening in a region just beyond the corotation radius and this becomes large as the spin equilibrium state is approached. It is suggested that this may lead to some mass loss from this region, resulting in the formation of a local centrifugally accelerated wind. The perturbation to the inner accretion rate and disc disruption radius is shown to change the equilibrium state and cause a short time‐scale torque reversal on the star, as is observed in some X‐ray binary pulsars. The effect is much weaker for an accreting white dwarf due to the relatively small values of density and temperature in the region beyond the corotation radius.