Long-term variations of the relative intensities of the violet and red components in
double-peaked emission line profiles are exhibited by many B
e stars.
Okazaki (
1991) explains
these variations as arising from a global-one-armed-oscillation in the disc, which appears as a
one-armed spiral pattern. The global disc oscillation is a perturbation of the disc, so that the
disc material is redistributed or “sloshed” about as the gas in the disc undergoes Keplerian
motion (
Ignace,
2000; see also
Okazaki,
2000). The variations are seen as evidence of a
non-axisymmetric emitting shell rotating about the star with the period of the V/R
variations (
Cowley and Gugula,
1973). Figure
1.6 (right panel) shows observations of H
profiles, of 59 Cygnus, which exhibits V/R variability in terms of relative flux vs
wavelength (
Doazan et al.,
1985). The left panel of Figure
1.6 shows a schematic
representation by
Telting et al. (
1994) of the theory proposed by
Okazaki (
1991); that a global
one armed oscillation in the disc is responsible for the observed V/R variations.
The observed V/R variations also show that the azimuthal velocity of the disc
is larger than the radial velocity, a necessary condition for sustaining the wave;
if the matter in the equatorial plane moved more quickly in the radial direction
than azimuthally the global disc oscillation would be unable to complete a full
circuit of the star and thus the waves giving rise to the V/R variations could not
grow.
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