Tidal Warping of Be Star Decretion Discs

Rapidly rotating Be stars are observed as shell stars when the decretion disc is viewed edge on. Transitions between the two implies that the discs may be warped and precessing. Type II X-ray outbursts are thought to occur when the warped disc interacts with the fast stellar wind. We suggest that tides from a misaligned companion neutron star can cause the observed effects. We make numerical models of a Be star decretion disc in which the spin of the Be star is misaligned with the orbital axis of a neutron star companion. Tidal torques from the neutron star truncate the disc at a radius small enough that the neutron star orbit does not intersect the disc unless the eccentricity or misalignment is very large. A magnetic torque from the Be star that is largest at the equator, where the rotation is fastest, is approximated by an inner boundary condition. There are large oscillations in the mass and inclination of the disc as it moves towards a steady state. These large variations may explain the observed changes from Be star to Be shell star and vice-versa and also the Type II X-ray outbursts. We find the tidal timescale on which the disc warps, precesses and reaches a steady state to be around a year up to a few hundred years. If present, the oscillations in mass and disc inclination occur on a fraction of this timescale depending on the orbital parameters of the binary. The timescales associated with the tidal torque for observed Be star binaries suggest that these effects are important in all but the longest period binaries.
Comment: Accepted for publication in MNRAS