Stellar wind mechanisms and instabilities

Abstract

I review driving mechanisms for stellar winds, using first the example of the coronal, pressure-driven solar wind, but then focussing mainly on radiation-pressure driven winds from hot, luminous stars.For the latter, I review the central role of line-opacity as a coupling between matter and radiation, emphasizing how the Doppler shift of an accelerating wind outflow exposes the strong line opacity toa substantial continuum flux, and thus allows the line force to sustain the outward acceleration against gravity.Through the CAK formalism that assumes a power-law distribution of line-opacity, I derive the mass loss rate and wind velocity law, and discuss how these are altered by various refinements like a finite-disk correction, ionization variations in opacity, and a non-zero sound speed.I also discuss how multiline scattering in Wolf-Rayet (WR) winds can allow them to exceed the single scattering limit, for which the wind and radiative momenta are equal. Through a time-dependent perturbation analysis, I show how the line-driving leads to a fast, inward "Abbott-wave" mode for long wavelength perturbations, and a strong Line-Deshadowing-Instability at short wavelengths, summarizing also 1D and 2D numerical simulations of the nonlinear evolution of this instability.I next discuss how rapid stellar rotation alters the latitudinal variation of mass loss and flow speed, and how this depends on treatment of gravity darkening, nonradial line forces, and "bi-stability" shifts in ionization.Finally, I conclude with a discussion of the large mass loss epochs of Luminous Blue Variable (LBV) stars, and how these might be modeled via super-Eddington, continuum driving moderated by the "porosity" associated with extensive spatial structure.