Dredge-up effects in galactic and magellanic planetary nebulae

Abstract

Chemical abundances are reported for 23 plan- etary nebulae in the Small Magellanic Cloud. Average abundances agree with values found for HII regions, sug- gesting that progenitors of most of these objects have been formed in the last 1 2 Gyr. The N/O vs. O/H anticorrelation is conrmed for Magellanic and galactic planetaries, and a more robust result is obtained if ob- jects of similar ages (or masses) are grouped together. For a given class, such a negative correlation suggests that the surface enrichment due to dredge-up episodes is more ecient at lower metallicities, in agreement with recent computations. CN-cycle products are dredge-up, oxygen remains un- changed, but some reduction is expected if the ON-cycle is operative. Theoretical estimates predict a maximum (un- certain) reduction of about 0.11 dex in the surface abun- dances of massive progenitors (Renzini & Voli 1981). The analyses of the C/N and C/O ratios in galactic super- giants suggest that these stars have dredged-up essentially CN-cycle products and that oxygen remained unchanged (Luck & Lambert 1985). Concerning PN, dierent data sets (Koppen et al. 1991; Kingsburgh & Barlow 1994; Costa & de Freitas Pacheco 1996) indicate that the oxy- gen abundances of massive and young planetaries (type I objects) agree with abundances derived from HII regions, consistent with the conclusion derived from galactic su- pergiants, namely, oxygen was not aected. These rst considerations may give some support to the scenario in which oxygen would not be altered by mixing episodes and its abundance reflects the pristine chemical composition of the medium from where PN have been formed out.