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Astron. Astrophys. 363, 717-732 (2000)


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A radio-continuum and photoionization-model study of the two planetary nebulae in the Sagittarius dwarf galaxy

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

1 Department of Physics and Applied Physics, University of Strathclyde, Scotland, UK (gregory.dudziak@strath.ac.uk)
2 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 Principal Cedex, France (daniel.pequignot@obspm.fr)
3 Department of Physics, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester, M60 1QD, UK (aaz@iapetus.phy.umist.ac.uk)
4 Space Telescope European Co-ordinating Facility, European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany (jwalsh@eso.org)

Received 24 February 2000 / Accepted 29 August 2000

Abstract

Radio continuum observations at 1.4, 4.8 and 8.6 GHz of the two Planetary Nebulae (PNe) in the Sagittarius dwarf galaxy reveal the elongated shape of Wray 16-423 and the extreme compactness of He 2-436. It is confirmed that He 2-436 is subject to local dust extinction.

Photoionization models for both PNe are obtained from two different codes, allowing theoretical uncertainties to be assessed. Wray 16-423, excited by a star of effective temperature 1.07[FORMULA] K, is an ellipsoidal, matter-bounded nebula, except for a denser sector of solid angle 15[FORMULA]. He 2-436, excited by a 7[FORMULA] K star, includes two radiation-bounded shells, with the very dense, low-mass, incomplete, inner shell possibly corresponding to a transitory event. The continuum jump at the He+ limit ([FORMULA]22.8 nm) agrees with NLTE model stellar atmospheres, despite the Wolf-Rayet nature of the stars. Both stars are on the same (H-burning) evolutionary track of initial mass (1.2[FORMULA]0.1) [FORMULA] and may be twins, with the PN ejection of Wray 16-423 having occured [FORMULA] 1500 years before He 2-436.

The PN abundances re-inforce the common origin of the parent stars, indicating almost identical depletions with respect to solar for O, Ne, Mg, S, Cl, Ar, and K (-0.55[FORMULA]0.07 dex), and strong overabundances for carbon, particularly in He 2-436. He[FORMULA] lines consistently point to large identical overabundances for helium in both PNe. An excess nitrogen makes Wray 16-423 nearly a Type I PN.

These PNe provide a means to calibrate both metallicity and age of the stellar population of Sagittarius. They confirm that the youngest, most metal-rich population has an age of 5 Gyr and a metallicity of [Fe/H][FORMULA], in agreement with the slope of the red giant branch.

Key words: stars: AGB and post-AGB – ISM: planetary nebulae: general – ISM: planetary nebulae: individual: He 2-436, Wray 16-423 – galaxies: dwarf – galaxies: individual: Sagittarius

Send offprint requests to: D. Péquignot

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000

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