Mass loss from evolved stars. II - Radio continuum emission and evolution to planetary nebulae

The radio continuum properties of cool evolved stars are investigated from the viewpoint of mass loss mechanisms and evolution through the planetary nebula stage. The VLA was used to search 32 high mass loss rate objects for 6-cm continuum emission, leading to the detection of NGC 7027, CRL 618, R Aqr, IRC + 10216, and Omicron Ceti. The evolution of such objects to the planetary nebula stage was studied by calculating the propagation of an ionization front into the remnant neutral envelope that is produced by continuous mass loss in the red giant stage. It is found that none of the stars has a hot, compact companion, except for the known cases of R Aqr and Mira, and that envelope fractional ionization is in most cases as small as less than about 0.001.