Molecules in dense globules in planetary nebulae

We have investigated the formation of molecules in a dense, neutral globule, such as those observed in the Helix nebula (NGC 7293). We find that H2 and CO can possess large abundances in cold gas throughout the bulk of the globule, in spite of the enhanced photon field provided by the central star. Emission from C2H and CN will be detectable from individual globules if they are carbon-rich. The temperature in the boundary layer between a globule and the wind of the nebula's central star is estimated, and found to be too low for neutral-neutral reactions to cause most boundary-layer oxygen to be in OH and H2O.