Mass loss from evolved stars. VI - Mass-loss mechanisms and luminosity evolution

A large body of molecular line observations of circumstellar envelopes produced by mass loss from highly evolved red giant stars is used to discuss mass-loss mechanisms for these stars. New observations are presented for several objects. Comparison of the derived mass-loss rates Mand outflow velocities V0 with the stellar luminosities L* and the optical depths of the dust envelopes strongly suggests that radiation pressure on dust drives the mass loss to its terminal velocity. The observations for the planetary and protoplanetary nebulae suggest that the stellar luminosity drops after the onset of planetary nebula formation, as predicted by stellar evolution theory. For a number of these objects, however, observational evidence suggests an alternative hypothesis, viz. that copious mass loss is the result of binary star interaction.