HYDRODYNAMICAL MODELS OF PLANETARY-NEBULAE AND THE PROBLEM OF ABUNDANCE DETERMINATIONS

The problem of the accuracy that can be attained in the derivation of chemical abundances in Planetary Nebulae (PNe) with the best technique s of the ''constant T-e, n(e)'' type currently in use, has been consid ered. This has been done by constructing two sequences (A, B) of radia tion-hydrodynamical PNe models under the framework of the interacting stellar-winds theory which are intended to represent real planetaries in different evolutionary stages. Both sequences evolve along the 0.60 5 M. stellar evolutionary track of Blocker (1995b), but start from dif ferent initial conditions. Corresponding equilibrium models were also computed for a number of specific models along the stellar track to ob tain an estimate of the errors that would result from stationary PNe m odels. The line intensities calculated from these models are interpret ed under the scheme of the ''constant T-e, n(e)'' method and the deriv ed elemental abundances are compared with the original input values. T he obtained deviations vary for the different elements. In the optical ly thin cases the discrepancies between the abundances derived using t he ''constant T-e, n(e)'' method and the corresponding input values am ount to less than 10% for helium and to maximum factors of 1.5 to 3 fo r oxygen, nitrogen, neon and argon. The discrepancy is higher for sulp hur, reaching an order of magnitude in the most excited models. Partic ular attention has to be given to optically thick models. It is found that the ''constant T-e, n(e)'' method can yield rather erroneous resu lts in these cases. Only minor deviations are found between abundances calculated from the hydrodynamical models and those in equilibrium.