Instabilities in massive stars

A defining property of massive stars is the dominant, dynamical role played by radiation throughout the stellar interior, atmosphere, and wind. Associated with this radiation hydrodynamics are several distinct kinds of instabilities that can lead to convection in both core and envelope, clumping in atmosphere and wind outflow, and perhaps even the dramatic mass loss outbursts associated with Luminous Blue Variable phases. Here I review these instabilities with emphasis on basic physical properties of radiative driving. I draw on two specific examples of dynamical instability, namely the strong instability associated with line-driving of a stellar wind outflow, and the global stellar instabilities associated with approaching or exceeding a modified Eddington limit. I conclude with a brief mention of recent ideas on the role of stellar rotation in the shaping of bipolar LBV outbursts. 1. Radiative force and the Eddington limit The cores of massive stars have long been recognized to be unstable to convection, and even to eventual collapse and supernova explosion once nuclear fuels have been exhausted. But in recent years there has been a growing appreciation that their envelopes, atmospheres, and winds may also be subject to a variety of instabilities, rooted generally in the strong dynamical role of radiative momentum in these characteristically very luminous stars. To focus the present review on these dynamical instabilities, let us begin by considering the general form for the radiative acceleration grad associated with a specific opacity κν (a.k.a. the mass absorption coefficient, with CGS units cm2 g−1) for interaction of stellar material with a radiative flux Fν at photon frequency ν,