Nonspherical expansion of radiation-driven winds

The line-driven wind theory of Castor et al. (1975) is extended to include effects arising from nonspherical expansion. Specifically, isothermal flow along the central streamline of a flow tube whose cross-sectional area increases outward faster than r-squared near the stellar surface is considered. Using the area function of Kopp and Holzer (1976), it is found that rapid flow tube divergence causes the critical point to occur closer to the stellar photosphere. Even modest departures from spherically symmetric expansion can significantly enhance the radiative force, and lead to large increases in the asymptotic flow speed of the wind. For models in which the region of rapid geometrical divergence is located more than a few tenths of a stellar radius from the photosphere, the mass flux density at the base of the wind is essentially unchanged from that of the spherical case. These results may have implications for the occurrence and structure of streams and corotating interaction regions in the winds of hot stars. 17 references