PHOTOIONIZATION MODELING OF MAGELLANIC CLOUD PLANETARY-NEBULAE .1.

We present the results of self-consistent photoionization modeling of a first sample of 38 Magellanic Cloud planetary nebulae (PN). From these models, we have constructed a Hertzsprung-Russell (H-R) Diagram for the central stars, and we have derived both the chemical abundances and the nebular parameters. We find that effective temperatures, T(eff), derived from a nebular excitation analysis, agree well with the temperatures derived by the classical Zanstra method. However, the nebular analysis method allows us to derive the effective temperatures of hot central stars, as well as in the case of the cooler stars to which the Zanstra method is generally restricted. We find a very good linear correlation between log (T(eff)) and the excitation class. From their positions on the H-R diagram, we conclude that the majority of the central stars in this sample have masses between 0.55 and 0.7 M. and are observed during their hydrogen-burning excursion towards high temperatures. Optically thin objects are found scattered throughout the H-R diagram, but tend to have a somewhat smaller mean mass. A few objects are found to have central stars with very high effective temperatures and masses in excess of 0.7 M.. These objects tend to be type I PN, with evidence of a third dredge-up episode resulting in correlated He and N abundance enhancements. The nebular mass of the optically thick objects is closely correlated with the nebular radius, and PN with nebular masses in excess of 1 M. are observed. The velocity of expansion of the nebula is very well-correlated with the position of the central star on the H-R diagram, and is evidence for continual acceleration of the nebular shell during the transition toward high T(eff). Excluding the type I PN, the mean abundances derived for the LMC and SMC agree very well with the mean abundances previously derived from observation of H II regions and evolved radiative supernova remnants.