Radiation driven instabilities in stellar winds

This thesis investigates the quantitative nature of the variability which is present in the stellar winds of high luminosity early type stars. A program of optical observations with high time and spectral resolution was designed to provide quantitative information on the nature of the fluctuations. These observations found no optical variability over a time period of six hours and hence restrict the variability over this period to size scales of less than 5x1011 cm, but do confirm the variations on time scales exceeding one day. A class of X-ray sources comprised of a neutron star orbiting a star with a strong stellar wind provides another source of information on the variability of stellar winds. A theory of accretion onto a neutron star was developed which is used with X-ray intensity data to derive estimates of the density and velocity of the stellar wind. This analysis performed on Cen X-3 suggests that the velocity in the stellar wind increases as the wind density increases. A theoretical analysis of the stability of a stellar wind is made to determine whether the variability may originate in the wind itself. Two types of instability are founds those that amplify pre-existing disturbances, and absolute instabilities which can grow from random motions within the gas. It is found that short wavelength disturbances (10¹¹ cm) are damped by radiation if the gas is thermally stable, that is if the net radiative energy loss increases with temperature. Intermediate wavelengths of about 10⁸⁻⁹ cm are usually subject to an amplification due to the density gradient of the wind. The radiation acceleration amplifies disturbances of scales 10⁷ to 10¹¹ ca. Absolute instabilities are present if the gas is thermally unstable, if the flow is deccelerating, or if the gas has a temperature of several million degrees. On the basis of the information derived on stellar wind stability it is proposed that a complete theory should be based on the assumption that the wind is a nonstationary flow.Science, Faculty ofPhysics and Astronomy, Department ofGraduat