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Paper: Links between Photospheric Activity and Formation of Circumstellar Structures of Be Stars (Invited Review)
Volume: 361, Active OB-Stars: Laboratories For Stellar and Circumstellar Physics
Page: 219
Authors: Rivinius, T.
Abstract: The circumstellar environment of Be stars is filled and shaped by the stars themselves through some still to be identified ejection process(es). Nevertheless the recent years have seen significant progress toward constraining these processes. In particular, it was shown that the two main candidates considered in the past decades, magnetic fields and nonradial pulsation, fall short of the goal aimed at, if considered as being the only contributor to this task. Only for a few stars a direct link between the photospheric activity and the ejected matter could be made. This includes the timing of the ejection of matter through pulsational beating, the observations of strongly blue-shifted short-lived absorption features, and transient V/R-variations. These variations possibly indicate orbiting inhomogeneities just after their ejection from the central star. However, multi periodic beating can be thoroughly excluded for some of the best investigated Be stars, and the direct observational evidence for magnetic ejections is sparser still. Single-mode pulsations alone are unable to transfer matter from a “canonical model Be star”, i.e. an early type B star rotating at about 80% of its critical rate; there is a substantial lack of energy and angular momentum. A traditional approach to solve this problem consists of adding new qualities such as binarity to the standard model, but such attempts have not led to overwhelming success. It seems much more promising to abolish the traditional quantitative barrier of 80% critical rotation, especially since recent calculations show this to be a viable – if not even necessary – option.
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