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 N ebulae (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 H e 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 1.07x10(5) K, is an ellipsoidal, matter-bou nded nebula, except for a denser sector of solid angle 15%. He 2-436, excit ed by a 7x10(4) K star, includes two radiation-bounded shells, with the ver y dense, low-mass, incomplete, inner shell possibly corresponding to a tran sitory event. The continuum jump at the He+ limit (lambda 22.8 nm) agrees w ith NLTE model stellar atmospheres, despite the Wolf-Rayet nature of the st ars. Both stars are on the same (H-burning) evolutionary track of initial m ass (1.2+/-0.1) M. and may be twins, with the PN ejection of Wray 16-423 ha ving occured similar to 1500 years before He 2-436. The PN abundances re-inforce the common origin of the parent stars, indicat ing almost identical depletions with respect to solar for O, Ne, Mg, S, CI, Ar, and K (-0.55+/-0.07 dex), and strong overabundances for carbon, partic ularly in He 2-436. He I Lines consistently point to large identical overab undances for helium in both PNe. An excess nitrogen makes Wray 16-423 nearl y a Type I PN. These PNe provide a means to calibrate both metallicity and age of the stel lar population of Sagittarius. They confirm that the youngest, most metal-r ich population has an age of 5 Gyr and a metallicity of [Fe/H]= -0.55, in a greement with the slope of the red giant branch.