Radiation pressure on grains as a mechanism for mass loss in red giants

Abstract

A quantitative model is constructed for the process of radiation pressure on dust to explain the mass loss observed in cool giants. Results are given for six sample stars, demonstrating the effects of stellar luminosity, mass, and effective temperature on the mass loss rate. The envelope expansion velocity and average grain size related to the fraction of grain material condensed and the gas density is found to be the key factor in determining the feasibility of mass ejection under this mechanism. Subject headings: late-type stars — mass loss — radiative transfer I. INTRODUCTION It has been known for nearly 4Q years that in most M giants, deep, narrow absorption lines are found displaced toward the violet from the normal broad absorption lines by about 2-25kms _1 . In 1956, Deutsch interpreted this displaced component as an indication of mass loss from the star after his analysis of the spectrum of the visual companion of a Her (Deutsch 1956). Since that time, many physical mechanisms have been proposed to explain this phenomenon. These include actions of turbulent motions, vaporization, shock waves, and electromagnetic acceleration. The turbulent velocities implied by the curve of growth are large compared with thermal motions in some cases, but fall far short of the escape velocities needed (Deutsch 1960). Rubbra and Cowling (1959) also demonstrated the insufficiency of other mechanisms. Radiation pressure has always been a popular source of power because of the abundance of energy available. However, Weymann (1962a) has shown that radiation pressure on atoms and molecules is probably inadequate. Hoyle and Wickramasinghe (1962) suggested that radiation pressure acting on dust may drive the gas to escape velocity. Recent infrared observations have indicated the presence of silicate material in the Trapezium region of M42 (Ney and Allen 1969) and in clouds surrounding certain cool stars (Woolf and Ney 1969). Gilman (1969) has shown that graphite and silicon carbide could arise from cool carbon stars, and iron and silicate particles may arise from cool, oxygen-rich stars. Hackwell (1972) found that carbon stars have a common emission feature at 10.8 /x which could possibly be due to silicon carbide. After the existence of circumstellar dust had been established, the mechanism of radiation pressure acting on grains became more plausible. So far this process has been only theoretically studied by ana-lytical approximations (Gehrz and Woolf 1971; Gilman 1972). In this paper we shall seek numerical solutions to the equation of motion by including the effects of (a) radiation pressure on grains; {b)