THE CHEMISTRY OF PROTQPLANETARY NEBULAE

摘要

We discuss three recent attempts at modelling the chemical evolution of the post-AGB phase of stellar evolution, concentrating mostly on the short-lived protoplanetary nebula (PPNe) phase. Although the three models discussed differ in their physical description of PPN, all agree that the high densities, temperatures and UV fluxes can drive a complex organic chemistry on a very short timescale. For instance, O atoms liberated from CO, either by pho-todissociation or by shock waves, can be converted into OH and H_2O, thereby producing oxygen-bearing molecules in a carbon-rich environment. However, once the molecular region becomes transparent to UV radiation, caused by geometrical dilution if this region is expanding, molecules are quickly destroyed; only the most photo-resistant species, and those which may exist in dense clumps, can survive to the planetary nebula phase.