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Astron. Astrophys. 361, L1-L4 (2000)

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Letter to the Editor

Third-dredge-up oxygen in planetary nebulae *

D. Péquignot  **  1, J.R. Walsh  ***  2, A.A. Zijlstra  ****  3 and G. Dudziak  *****  4

1 Laboratoire d'Astrophysique Extragalactique et de Cosmologie associé au CNRS (UMR 8631) et à l'Université Paris 7, DAEC, Observatoire de Paris-Meudon, 92195 Meudon Cédex, France
2 Space Telescope European Co-ordinating Facility, European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching bei München, Germany
3 University of Manchester Institute of Science and Technology, Department of Physics, P.O. Box 88, Manchester, M60 1QD, UK.
4 Department of Physics and Applied Physics, University of Strathclyde, Scotland

Received 22 March 2000 / Accepted 14 August 2000

Abstract

The planetary nebulae He 2-436 and Wray 16-423 in the Sagittarius dwarf galaxy appear to result from nearly twin stars, except that third-dredge-up carbon is more abundant in He 2-436. A thorough photoionization-model analysis implies that ratios Ne/O, S/O and Ar/O are significantly smaller in He 2-436, indicative of third-dredge-up oxygen enrichment. The enrichment of oxygen with respect to carbon is (7[FORMULA]4)%. Excess nitrogen in Wray 16-423 suggests third dredge-up of late-CN-cycle products even in these low-mass, intermediate-metallicity stars.

Key words: nuclear reactions, nucleosynthesis, abundances – ISM: planetary nebulae: general – ISM: planetary nebulae: general – ISM: planetary nebulae: general – galaxies: dwarf – galaxies: individual: Sagittarius

* Based on observations collected at ESO La Silla, Chile.
** e-mail: daniel.pequignot@obspm.fr
*** e-mail: jwalsh@eso.org
**** e-mail: aaz@iapetus.phy.umist.ac.uk
***** e-mail: gregory.dudziak@strath.ac.uk

Send offprint requests to: D. Péquignot

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000
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