CHEMICAL EVOLUTION FROM THE AGE TO THE PLANETARY-NEBULA PHASE

An overview is given on the wealth of data recently provided by large mm-wave radiotelescopes on AGE stars, planetary nebulae (PNe), and tra nsition objects. The observations reveal that there is an observable c hemical evolution in the neutral gas as a star evolves beyond the AGE, through the proto-PN and PN phases. Significant changes in the abunda nces of some key molecules (such as CS, CN: HCO+, HCN, and HC3N) take place during the fast evolution of the envelopes. Chemistry can thus b e used as a rough clock to date the evolutionary stage of post-AGE env elopes and proto-PN objects. However, once the PN is formed, the obser ved abundances in the molecular clumps of the envelope remain relative ly constant. The chemical evolution of the molecular envelopes likely occurs through the development of photon-dominated regions produced by the ultraviolet held of the central star. The main chemical processes which likely control the evolution are also reviewed.