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, SiC , CS, and HC N decrease, and they are not detected in the PNe, while the abundances of CN, HNC, and 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 , and the average abundance ratios are CN/HCN = 9, HNC/HCN = 0.5, and /HCN = 0.5. These ratios are, respectively, one, two, and three orders of magnitude higher than in the prototypical AGB envelope IRC+10216. The ratios are , 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 show that the clumps which form the envelopes of the
evolved PNe maintain remarkably high gas densities (
few cm ) and low temperatures ( K).
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
Accepted by Astron. Astrophys.
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