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

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 16423 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 1.07×10K, is an ellipsoidal, matterbounded nebula, except for a denser sector of solid angle 15%. He 2-436, excited by a 7×10K 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 (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±0.07 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.