Turbulence in Line-Driven Stellar Winds

Summary

Hot, luminous OB stars have strong stellar winds driven by the line-scattering of the star's continuum radiation. This line-driving mechanism is understood to be highly unstable to small scale perturbations. I review efforts to simulate the nonlinear evolution of this instability using radiation hydrodynamics simulation codes. Because the usual local, Sobolev treatment for the line-force does not apply, a major challenge has been to develop computationally tractable methods for approximating the inherently non-local radiative transfer in the large number of wind-driving lines. Results of 1-D simulations generally show development of a highly compressible, stochastic wind structure dominated by strong reverse shocks and dense shells; these arise from amplification of inward-propagating radiatively-modified acoustic modes with anticorrelated velocity and density. In 2-D and 3-D, linear analysis predicts that lateral variations in velocity should be strongly damped by the “line-drag” effect of the diffuse radiation scattered within the line resonance, suggesting possible suppression of classical Rayleigh-Taylor modes for lateral breakup of wind structure. This motivates current efforts toward 2-D simulation of the nonlinear wind structure. An overall goal is to develop connections with studies of highly compressible turbulent structure in other physical and astrophysical contexts.

Introduction

The focus of this meeting is on Interstellar turbulence, but the scientfic program reflects the importance of examining this in a general context, including earlier studies of incompressible turbulence in terrestrial fluids, as described in the classical theory of Kolmogorov and its modern extensions (Frisch 1995).