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| Paper: |
Warping of Be Star Decretion Discs in X-Ray Binaries |
| Volume: |
447, Evolution of Compact Binaries |
| Page: |
251 |
| Authors: |
Martin, R. G.; Pringle, J. E.; Tout, C. A.; Lubow, S. H. |
| Abstract: |
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. It is thought that Type II X-ray outbursts
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
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. |
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