Nonradial pulsations of O- and B-stars

Abstract

The available observational facts are reviewed which show that in the hot, high-luminosity corner of the HR-diagram nonradially pulsating stars are very common. For individual stars a proof of nonradial pulsation is not always easy to furnish, but the global evidence is very strong. With the possible exception of supergiants, the OB-star instability region is separated from other nonradially pulsating stars such as the δ Scuti stars by a zone starting around B7 where even low-amplitude pulsators suddenly become rare. OB-stars are also theoretically much less elusive than has sometimes been suggested in the past since the recent upward revision of stellar opacities now enables driving of the pulsations also by the classical κ-mechanism possible. Within the OB-domain, sub-patterns of pulsational properties begin to show up in the observational data. As an example with other interesting repercussions, some attention is given to the double-dichotomy between Be- and Bn-stars. Both are rapid rotators and among B-stars earlier than B7 roughly equally abundant. But the incidence of low-order nonradial pulsation modes is much higher in Be-stars, and only Be-stars go through outburst-like phases of strongly enhanced mass loss rates. Interdependencies between (a) rapid rotation and chemical structure, (b) abundances and pulsation, and (c) pulsation and rotation might in some combination lead to these two different B-star sub-populations. Observational possibilities to discriminate between various alternatives are briefly discussed.