SIMBAD references

We study the chemistry in the slow wind during the transition from asymptotic giant branch (AGB) star to protoplanetary nebula (PPN) and planetary nebula (PN). We show that there is a very rich chemistry of degradation products created by photoprocessing, driven by the gradually hardening radiation field of the central star. Most of these products are, however, photodissociated during the PPN phase if the wind is smooth. By contrast, if the wind is clumpy, possibly because of clumpiness in the AGB atmosphere, then many of these degradation products survive into the PN phase. Thus, chemistry may be used to infer the existence of clumpiness in the AGB phase. We identify potential molecular tracers, and we note that, in the case of clumpiness, large molecules may survive the transport from the stellar atmosphere to the interstellar medium. We compare between our model results with observations of three objects at differing evolutionary stages: CRL618, NGC 7027 and the Helix nebula (NGC 7293).