MODELS OF PLANETARY-NEBULA SPECTRAL EVOLUTION

First results are presented for models of the overall spectral evolution of planetary nebulae, with emphasis on the far-infrared emission of the remnant asymptotic giant branch (AGB) dust shells. The models combine a stellar/nebular spectrum, either an approximate model assuming the nebula to be strongly ionization-bounded or a proper photoionization model, with a dust radiative transfer calculation. A series of these calculations are used to follow the evolution of the spectrum from the end of the AGB through to the white dwarf cooling stage. Combining these spectral results with an assumed Galactic distribution of planetary nebulae allows the simulation of a large number of observational quantities including the nebular V-magnitude, H-beta flux, the IRAS colors, and the 5 GHz radio flux density. Those models using the full photoionization calculation also yield simulated sky image profiles at radio wavelengths and in some optical forbidden lines. Comparison of the model results to the IRAS color-color diagram indicate that the Schonberner 0.64 M. central star model evolves too slowly to match the observations. The first results indicate, still with some uncertainty, that a range of initial 10-mu-m dust optical depths from 5 to 20 plus a faster evolving central star gives a reasonable match to the IRAS colors. The predicted evolution of the 8 to 23-mu-m spectrum qualitatively resembles the available IRAS low-resolution spectrometer data. The models indicate that sky images should be able to clearly distinguish between matter-bounded and ionization-bounded nebulae and between the interacting winds model and the superwind model of planetary nebula formation, distinctions which cannot be easily made based solely upon the optical spectrum.