GALACTIC PLANETARY-NEBULAE WITH WOLF-RAYET NUCLEI - I - OBJECTS WITH [WC]-EARLY TYPE STARS

Spatially resolved long-slit spectrophotometric data for the planetary nebulae PB6, NGC 2452, NGC 2867, NGC 6905 and He 2-55 are presented. Different knots were observed in each nebula. All the nebulae are ioni zed by [WC 2-3] type nuclei. For the five objects, we calculated photo ionization models using the ionizing radiation field from models of ex panding atmospheres. The photoionization models, built with the condit ion that the predicted stellar visual magnitude is equal to the observ ed one, were rather successful in reproducing at the same time the ion ization structure and the electron temperature of the nebulae, using m odel atmospheres that were close (+/-20 000 K) to the best fit for rep roducing the stellar features, as presented by Koesterke & Hamann (199 7a). The constraints for the modelling procedure were to reproduce the observed intensity ratios of important lines of different ionization stages, and to be roughly consistent with the observed H beta flux, an gular diameter and morphology of the nebulae. We found that, for some objects, only two-density models with an inner zone of lower density c an meet all these requirements. These density structures are consisten t with the morphology showed by the nebulae. In a couple of cases, our photoionization modelling seems to indicate that the models of expand ing atmospheres used could be lacking ionizing photons with respect to their emission in the V band. Chemical abundances in the nebulae were derived from the ionic abundances observed and ionization correction factors obtained from the models. We found that, while the five nebula e of our program have very similar exciting stars (similar stellar tem peratures, mass loss rates, chemical compositions), the nebular chemic al compositions are different. PB 6 and NGC 2452 are He-, N-, and prob ably C-rich nebulae, indicating massive progenitors (M-initial greater than or equal to 2.8 M.). In particular, abundances in PB 6 are consi stent with a scenario of C produced via the triple-alpha process, bein g brought to the surface by the third dredge-up event and partially co nverted into N through envelope-burning. The other nebulae present typ ical disk-PNe abundances, showing only C enrichment (C/O greater than or equal to 1). Therefore their progenitors were not massive, but all underwent the third dredge-up. Thus, clearly, post-AGE stars of quite different initial masses can pass through a [WC] stage with similar at mospheric parameters. We did not find evidence for abundance variation s inside any of the nebulae. In PB 6 and NGC 2867, we found that the C /O ratios derived from the C III 1909/[O III] 5007 line ratios would i nduce electron temperatures significantly lower than observed. The dis crepancy would be larger if carbon abundances derived from the optical C II 4267 recombination lines are considered.