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Paper: A Working Hypothesis about the Cause of Be Stars: Episodic Outward Leakage of Low-Frequency Modes Excited by the Iron-Peak κ-Mechanism
Volume: 479, Progress in Physics of the Sun and Stars
Page: 325
Authors: Ishimatsu, H.; Shibahashi, H.
Abstract: We propose a working hypothesis to explain the Be-star phenomenon, by taking account of the effect leaky waves have on angular momentum transfer. If low-frequency non-axisymmetric nonradial oscillations are excited by the iron-bump κ-mechanism, such oscillations transport angular momentum from the driving zone to a damping zone. As a consequence, angular momentum is deposited near the surface due to wave dissipation, while the surface rotation velocity remains below the break-up velocity. However, this will result in a gradual increase in the “critical frequency for g-modes,” leading eventually to g-modes leaking outward long before the stellar surface rotation velocity reaches the break-up limit. Once nonradial oscillations start to leak out, angular momentum is lost from the star and a circumstellar disk is formed. On the other hand, the oscillation itself will soon be damped owing to kinetic energy lost overcoming the excitation mechanism. Once angular momentum is lost from the star, the stellar envelope spins down and angular momentum loss promptly stops. The star returns to being quiet and remains quiescent until new nonradial oscillations of sufficient amplitude are built up by the κ-mechanism and a new episode begins. According to this scenario, the interval of episodic Be-star activity corresponds to the oscillation growth time.
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