Chemical Evolution from the AGB to the Planetary Nebula Phase

An overview is given on the wealth of data recently provided by large mm-wave radiotelescopes on AGB stars, planetary nebulae (PNe), and transition objects. The observations reveal that there is an observable chemical evolution in the neutral gas as a star evolves beyond the AGB, through the proto-PN and PN phases. Significant changes in the abundances of some key molecules (such as CS, CN, HCO+, HCN, and HC3N) take place during the fast evolution of the envelopes. Chemistry can thus be used as a rough clock to date the evolutionary stage of post-AGB envelopes and proto-PN objects. However, once the PN is formed, the observed abundances in the molecular clumps of the envelope remain relatively constant. The chemical evolution of the molecular envelopes likely occurs through the development of photon-dominated regions produced by the ultraviolet field of the central star. The main chemical processes which likely control the evolution are also reviewed. This revised version was published online in September 2006 with corrections to the Cover Date.