1997A&A...324.1123B

We report millimeter line observations of CO, ¹³CO, SiO, SiC₂, CN, HCN, HNC, HCO⁺, CS, and HC₃N to study the chemistry in planetary nebulae (PNe) with massive envelopes of molecular gas. The sample observed consists of representative objects at different stages of development in order to investigate evolutionary effects: the proto-PNe CRL 2688 and CRL 618, the young PN NGC 7027, and the evolved PNe NGC 6720 (the Ring), M4-9, NGC 6781, and NGC 7293 (the Helix). The observations confirm that the chemical composition of the molecular gas in PNe is radically different from that in interstellar clouds and the circumstellar envelopes of Asymptotic Giant Branch (AGB) stars. There are also clear trends in the chemical evolution of the envelopes. As a star evolves beyond the AGB, through the proto-PN and PN phases, the abundances of SiO, SiC₂, CS, and HC₃N decrease, and they are not detected in the PNe, while the abundances of CN, HNC, and HCO⁺ increase dramatically. Once a PN has formed, the observed abundances in the molecular clumps of the envelope remain relatively constant, although HNC is anomalously underabundant in NGC 7027. In the evolved PNe, CN is about an order of magnitude more abundant than HCN, HNC, and HCO⁺, and the average abundance ratios are CN/HCN=9, HNC/HCN=0.5, and HCO⁺/HCN=0.5. These ratios are, respectively, one, two, and three orders of magnitude higher than in the prototypical AGB envelope IRC+10216. The ¹²C/¹³C ratios are ≃10-25, within the large range found in AGB envelopes. The chemical evolution of the envelopes likely occurs through the development of photon-dominated regions produced by the ultraviolet radiation field of the central star. The observations also provide important information on the physical conditions in the molecular gas. Multi-line observations of CN, CO, and HCO⁺ show that the clumps which form the envelopes of the evolved PNe maintain remarkably high gas densities (∼ few x 10⁵cm^–3) and low temperatures (∼25K). These values are consistent with the idea that the clumps are in rough pressure equilibrium with the more diffuse, ionized gas and can last for a significant part of the nebular lifetime, providing the environment needed for the survival of the molecules. Thus the clumping of the gas in these PNe is an essential aspect of both their physical and chemical evolution.