Stellar wind mechanisms and instabilities

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 to a 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.