Elemental abundances for a sample of southern galactic planetary nebulae.

We present spectrophotometric observations of a sample of 80 southern galactic planetary nebulae (PN), and derive elemental abundances for 68 objects, supplementing the optical observations with Uv data in 25 cases. We define Type I PN as those objects that have experienced envelope-burning conversion to nitrogen of dredged- up primary carbon. Such nebulae are recognised by their having nitrogen abundances that exceed the total C + N abundance of H ii regions in the same galaxy. In our own galaxy, this criterion corresponds to N/O >0.8. In the current sample, 11 nebulae having N/O >0.8 are thereby classified as Type I. For these Type I PN, no evidence is found for oxygen depletion, compared with non-Type I PN. No trend is found between the N/O and 0/H ratios for the entire sample, and the mean 0/H ratios for the non-Type I and Type I PN are the same within the errors; O/H=(4.93 12.22) X 10- by number for 42 non-Type I PN and O/H=(4.42 I 1.44) x 10- for 11 Type I PN. Also, no difference is found between the oxygen abundances in the PN in this sample and the oxygen abundances in galactic H ii regions. Hence we find no evidence for the ON cycle (which is predicted to operate during the second dredge-up) to have significantly altered the surface abundances of the progenitor stars, even for the Type I PN. The helium abundances derived for the non-Type I PN are in accord with those predicted by Becker & Iben for the first and third dredge-up phases. A comparison between the nitrogen abundances in the PN and the carbon + nitrogen abundances in galactic H ii regions indicates that roughiy 36 per cent of the initial carbon is converted into nitrogen in the case of the non-Type I PN, consistent with predictions for the first dredge-up by Becker & Iben. In order to explain the high nitrogen abundances derived for the Type I PN, however, envelope-burning of dredged-up carbon into nitrogen, following the third dredge-up, is definitely required. Total C + N +0 abundances are found to be correlated with C/H for the combined nonType I and Type I sample; the carbon has been enhanced by He-burning processed material brought up by the third dredge-up. Key words: stars: abundances - planetary nebulae: general.