Origin of winds in cool giants and supergiants

The properties of the winds of cool giants and supergiants are reviewed. The observations of circumstellar lines, dust shells, absorption lines in the spectra of companions, resonance-scattered light, molecular absorption, interferometry, and UV, X-ray and radio fluxes are discussed with respect to finding the mass loss rate of the star. Four classes of model for the origin of the wind are then considered: dust-driven models, gas-pressure-driven models, wave-pressure-driven models, and shock wave-driven models. All the models have inadequacies, so the actual origin of the winds is not clear. (Auth.)