Mechanisms for mass loss from cool stars

Abstract

The mechanisms believed responsible for the loss of mass from cool, red giant stars are reviewed. While observations indicate that both radiation pressure on dust grains and pulsations are important, theoretical considerations indicate that neither is sufficient by itself to account for the high rates of mass loss that have been observed. The current picture involves a two-step process wherein pulsations act to levitate matter well above the photosphere to the point at which the gas is sufficiently cool for dust grains to form. Radiation pressure on the dust then drives the matter to infinity. Whereas this model is applicable to spherically symmetric mass loss, the outflowing matter in many mass-losing systems displays a pronounced bipolarity, implying axial symmetry on the large scale. A secondary star appears to be responsible for the geometry of such systems. A new scenario involving two winds is presented to describe how the bipolar geometry might be produced.