Be Stars as Nonradial Pulsators (Review Paper)

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Abstract
The observational status of the ubiquitous rapid variability of Be stars is summarized. The most comprehensive interpretation is obtained with a traveling velocity field and associated temperature variations. But neither the available observations nor theoretical predictions presently allow an unambiguous mode determination of these nonradial pulsations. In addition to rapid rotation, an NRP mode of low azimuthal order, m ≈ 2, seems another prerequisite for a B star becoming a Be star. The amplitudes of these modes are variable and have been observed to decrease with some delay after an outburst. For low-order sectorial modes, NRP's therefore have the potential of explaining the three oldest unsolved problems of Be stars: the difference between Be and Bn stars (the latter do not usually have a low-order mode), the episodic component of the mass loss from Be stars, and the equatorial density maximum of their winds. Mechanisms for the transfer of energy to the atmosphere and its transformation into the kinetic energy of an outburst are discussed.