2001A&A...366..229H

The existence of warm zones of an enhanced molecular content surrounding the photospheres of AGB stars has been postulated to explain observed molecular absorption/emission components which are in excess of the results obtained from classical hydrostatic model atmospheres. We have analyzed the chemical equilibrium molecular composition of time-dependent models of carbon-rich circumstellar envelopes for various combinations of the stellar parameters and found that groups of molecules appear to be present in different zones with temporally varying temperature and density. The regions of enhanced molecular abundances are situated between the stellar photosphere and the circumstellar dust shell. In the models, these zones are produced by the levitation of the atmospheric gas due to the dissipation of shock waves and/or due to radiation pressure on molecules and dust.