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The chemical evolution of planetary nebulae

R. Bachiller tex2html_wrap_inline1451 , T. Forveille tex2html_wrap_inline1453 , P. Huggins tex2html_wrap_inline1455 , & P. Cox tex2html_wrap_inline1495
tex2html_wrap_inline1451 Observatorio Astronómico Nacional (IGN), Apartado 1143, E-28800 Alcalá de Henares, Spain
tex2html_wrap_inline1453 Observatoire de Grenoble, B. P. 53X, F-38041 Grenoble Cedex, France
tex2html_wrap_inline1455 Physics Department, New York University, 4 Washington Place, New York NY 10003, USA
tex2html_wrap_inline1457 Institut d'Astrophysique Spatiale, Bât. 121, Université de Paris XI, F-91405 Orsay Cedex, France
tex2html_wrap_inline1505 Institut d'Astrophysique de Paris, 92b bd. Arago, F-75014 Paris, France
Abstract: We report millimeter line observations of CO, tex2html_wrap_inline1343 , SiO, SiC tex2html_wrap_inline1303 , CN, HCN, HNC, tex2html_wrap_inline1511 , CS, and HC tex2html_wrap_inline1305 N 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, SiC tex2html_wrap_inline1303 , CS, and HC tex2html_wrap_inline1305 N decrease, and they are not detected in the PNe, while the abundances of CN, HNC, and tex2html_wrap_inline1511 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 tex2html_wrap_inline1511 , and the average abundance ratios are CN/HCN = 9, HNC/HCN = 0.5, and tex2html_wrap_inline1511 /HCN = 0.5. These ratios are, respectively, one, two, and three orders of magnitude higher than in the prototypical AGB envelope IRC+10216. The tex2html_wrap_inline1525 ratios are tex2html_wrap_inline1527 , 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 tex2html_wrap_inline1511 show that the clumps which form the envelopes of the evolved PNe maintain remarkably high gas densities ( tex2html_wrap_inline1347 few tex2html_wrap_inline1533  cm tex2html_wrap_inline1535 ) and low temperatures ( tex2html_wrap_inline1537  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 chemical evolution.
Accepted by Astron. Astrophys.
Preprints can be obtained by contacting bachiller@oan.es


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Next: The Gravitational Lensing Nature Up: Scientific results Previous: The extended 1.3 mm continuum

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