A new look at carbon abundances in planetary nebulae. IV. Implications forstellar nucleosynthesis

This paper is the fourth and final report on a project designed to study ca rbon abundances in a sample of planetary nebulae representing a broad range in progenitor mass and metallicity. We present newly acquired optical spec trophotometric data for three Galactic planetary nebulae, IC 418, NGC 2392, and NGC 3242, and combine them with UV data from the IUE Final Archive for identical positions in each nebula to determine accurate abundances of He, C, N, O, and Ne at one or more locations in each object. We then collect a bundances of these elements for the entire sample and compare them with the oretical predictions of planetary nebula abundances from a grid of intermed iate-mass star models. We find some consistency between observations and th eory, lending modest support to our current understanding of nucleosynthesi s in stars below 8 M . in birth mass. Overall, we believe that observed abu ndances agree with theoretical predictions to well within an order of magni tude but probably not better than to within a factor of 2 or 3. However, ev en this level of consistency between observation and theory enhances the va lidity of published intermediate-mass stellar yields of carbon and nitrogen in the study of the abundance evolution of these elements.