The instability of radiation-driven stellar winds

The stability of a radiation-driven stellar wind is examined by applying a completely linearized stability analysis to a physically realistic wind model. Three types of instability are found. Both the line-shape and the radiation-driven sound wave instabilities are purely vertical motions which have been previously discovered. A new 'gradient' instability has motion in both the vertical and horizontal directions. Typical growth times of the three instabilities are 150, 1000, and 10,000 seconds, respectively, although the growth time of the line-shape instability is a lower limit. The radiation-driven sound wave and gradient instabilities have density and velocity perturbations suggesting that the wind will tend to break up into clouds or slabs of typical size one-billion to 100-billion cm. The existence of such clumps would alter the dynamics of a wind and the interpretation of observations of winds.