Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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.

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The categories of this paper are: econ
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The categories of this paper are: chem-ph
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The categories of this paper are: math
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The categories of this paper are: cmp-lg
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The categories of this paper are: dg-ga
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The categories of this paper are: eess
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The categories of this paper are: q-fin
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The categories of this paper are: ao-sci
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The categories of this paper are: adap-org
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The categories of this paper are: atom-ph
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The categories of this paper are: solv-int
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The categories of this paper are: physics
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The categories of this paper are: patt-sol
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The categories of this paper are: funct-an
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The categories of this paper are: cond-mat
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The categories of this paper are: plasm-ph