ELEMENTAL ABUNDANCES FOR A SAMPLE OF SOUTHERN GALACTIC PLANETARY-NEBULAE

We present spectrophotometric observations of a sample of 80 southern galactic planetary nebulae IPN), and derive elemental abundances for 6 8 objects, supplementing the optical observations with UV data in 25 c ases. We define Type I PN as those objects that have experienced envel ope-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 sa mple, 11 nebulae having N/O > 0.8 are thereby classified as Type I. Fo r 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 O/H ratios for the entire sample, and the mean O/H ratios for the non-Type I and Type I PN are the same within the errors; O/H = (4.93 +/- 2.22) x 10(- 4) by number for 42 non-Type I PN and O/H = (4.42 + 1.44) x 10(-4) for 11 Type I PN. Also, no difference is found between the oxygen abundan ces in the PN in this sample and the oxygen abundances in galactic HII regions. Hence we find no evidence for the ON cycle (which is predict ed to operate during the second dredge-up) to have significantly alter ed 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 HII regions indicates tha t roughly 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 nitrog en abundances derived for the Type I PN, however, envelope-burning of dredged-up carbon into nitrogen, following the third dredge-up, is def initely required. Total C+N+O abundances are found to be correlated wi th C/H for the combined nonType I and Type I sample; the carbon has be en enhanced by He-burning processed material brought up by the third d redge-up.