2000A&A...363..717D

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 1.07x10⁵K, is an ellipsoidal, matter-bounded nebula, except for a denser sector of solid angle 15%. He 2-436, excited by a 7x10⁴K 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 (λ22.8nm) 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±0.07dex), and strong overabundances for carbon, particularly in He 2-436. HeI 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.