The chemistry of protoplanetary nebulae

We describe the chemistry occurring in a dense slab of outwardly expanding gas in the post-AGB phase of stellar evolution. Despite the high flux of UV photons from the central star, the intrinsic dust extinction in the slab prevents rapid photodissociation and allows chemical reactions to make a variety of gas-phase species, some complex. We find that chemical evolution ends when the intrinsic extinction falls below about 10 mag. At this point, rapid photodissociation occurs. As a particular example of our model, we discuss chemical synthesis in the protoplanetary nebula, CRL 618, and find that for many species we obtain good agreement between the model and observation, despite the complexity of the source.