Photoionization models of both nebulae are obtained from two different codes and different approaches, allowing theoretical uncertainties to be assessed. Wray 16-423 is modeled as an ellipsoidal, relatively optically thin nebula, except for a 15$\%$ radiation-bounded, denser sector, excited by a star of effective temperature 1.07 105K. He 2-436 is modeled as a double-shell nebula with a high-density, radiation-bounded, incomplete inner shell and an outer lower-density region, excited by an 0.70 105K star. The continuum jump at the He+ ionization limit is found to nearly agree with standard NLTE model stellar atmospheres, despite the Wolf-Rayet nature of the central stars.
Improved stellar parameters indicate that both central stars may belong to the same (H-burning) evolutionary track and have parent-star masses close to 1.20 solar masses, implying they belong to a relatively young population. It is suggested that the very dense inner shell presently seen in He 2-436 corresponds to a transitory phase and that these PNe constitute twin events with the PN ejection of Wray 16-423 having occured about 1500 years before He 2-436.
A reassessment of abundances re-inforces the common origin of the parent stars, indicating nearly identical depletions with respect to solar for N, O, Ne, Mg, S, Cl, Ar, and K [(-0.55 +/- 0.07) dex], similar overabundances for helium, strong overabundances for carbon (larger in He 2-436), and a relative excess of nitrogen in Wray 16-423.
If, as seems probable, these PNe (Z approximately 0.004) arise in the more
metal-rich stellar population of Sagittarius, then they provide a means
to calibrate the metallicity of the stellar population from photometry.
paper (postscript, 565k)
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