ELEMENTAL ABUNDANCES FOR A SAMPLE OF SOUTHERN GALACTIC PLANETARY-NEBULAE.
MON NOT R ASTRON SOC
257 - 299.
We present spectrophotometric observations of a sample of 80 southern galactic planetary nebulae IPN), 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 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 abundances 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 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 HII regions indicates that 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 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+O 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.
|Title:||ELEMENTAL ABUNDANCES FOR A SAMPLE OF SOUTHERN GALACTIC PLANETARY-NEBULAE|
|Keywords:||STARS, ABUNDANCES, PLANETARY NEBULAE, GENERAL, EFFECTIVE COLLISION STRENGTHS, STRUCTURE FORBIDDEN TRANSITIONS, GIANT BRANCH EVOLUTION, H-II REGIONS, CENTRAL STAR, 3P3 CONFIGURATION, O-III, SPECTROPHOTOMETRIC SURVEY, ELECTRON TEMPERATURES, CHEMICAL-COMPOSITIONS|
|UCL classification:||UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Physics and Astronomy|
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