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

Dudziak, G. and Pequignot, D. and Zijlstra, A.A. and Walsh, J.R. (2000) A radio-continuum and photoionization-model study of the two planetary nebulae in the Sagittarius dwarf galaxy. Astronomy and Astrophysics, 363 (2). pp. 717-723. ISSN 0004-6361

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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 ofWray 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 1.07×105K, is an ellipsoidal, matterbounded nebula, except for a denser sector of solid angle 15%. He 2-436, excited by a 7×104K star, includes two radiation- bounded shells, with the very dense, lowmass, incomplete, inner shell possibly corresponding to a transitory event. The continuum jump at the He+ limit (_22.8nm) 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±0.1) M⊙ and may be twins, with the PN ejection of Wray 16-423 having occured _ 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±0.07 dex), and strong overabundances for carbon, particularly in He 2-436. He i 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 5Gyr and a metallicity of [Fe/H]= −0.55, in agreement with the slope of the red giant branch.