The Origin and Development of Instabilities in Radiatively-Driven Stellar Winds

The numerous observational indicators of instability in the radiatively-driven winds of hot stars are review briefly, followed by a summary of the present theoretical understanding of the linear instability of such winds. This provides the motivation for the hydrodynamic simulation, the major thrust of the paper. A serious approximation that must be made in order to reduce the cost of the simulations to a reasonable level--the absorption approximation for the radiation force--is discussed in some detail. The hydrodynamic methods are described briefly, and then the computational results for winds models computed in the absorption approximation are discussed. The most notable results pertain to the critical nature of the ratio v{sub th}/a of the intrinsic line width to the sound speed. When this ratio is large, only a negligible wind results; when the ratio is small, the wind executes permanent self-excited oscillations; in an intermediate range the wind is globally stable, but acts as a powerful wave amplifier. The morphology of the oscillations--strong rarefactions and reverse shocks--is described and related to Abbott's linear theory, and the possible connection to observations is mentioned. 30 refs