4 Conclusions

We have performed new spectroscopic observations for a sample of 23 PN in the SMC and derived chemical abundances for He, N, O, Ne, S and Ar. Mean abundances agree with values observed in HII regions, suggesting that the bulk of these objects is young, with ages below 3 Gyr. Chemical evolutionary models indicate an enrichment of about 0.20 dex only, in the past 5 Gyr. This is comparable to the rmsd derived when computing sample averages.

We confirm that galactic and Magellanic PN show an anti-correlation between the N/O ratio and the oxygen abundance. Such a trend is more evident when planetaries are classified according to the mass range of the progenitors. This "grouping'' effect is clearly seen in galactic and in LMC objects. It should also be present in SMC planetaries, but the present sample is not complete, since older PN are probably missing.

If we discard oxygen conversion into nitrogen for the reasons already mentioned, then the most plausible explanation for such a negative correlation is that besides the mass of the progenitor, the metallicity must affect the efficiency of the dredge-up. Integrated yield contributions from low- and intermediate-mass stars, according to Renzini & Voli (1981), indicate that the amount of fresh ⁴He is almost constant with metallicity, whereas a small positive trend is derived for ¹⁴N. More recent calculations by Marigo (1999) suggest an opposite behavior: both ⁴He and ¹⁴N show decreasing integrated yields as the metallicity increases, in agreement with our findings. In the case of ⁴He, Marigo (1999) obtains a higher efficiency not only for decreasing metallicities, but also for increasing stellar masses. The ¹⁴N behavior is more complex: for a given metallicity, there is a stellar mass limit above which the efficiency increases abruptly. This mass limit decreases with metallicity approximately as log $M_{{\rm lim}} \approx 0.66 + 0.28$ log $(Z/Z_{\odot})$.

As a consequence of such a complex enrichment scenario, one should be cautious when classifying extra-galactic planetaries on the basis of chemical abundances only. Reversing the argument, the observed trend of LMC planetaries in the N/O - O/H plane allowed the identification of a group of objects, whose progenitors were formed at about 5 Gyr ago, suggesting that such a diagram could be an additional tool to investigate extra-galactic PN.

Acknowledgements

This work was partly supported by the brazilian agencies FAPESP (proc. 90/03803-3) and CNPq (proc. 300.818/93-9). T.P.I. acknowledges FAPESP for the present post-doc grant 97/13083-7 at IAG.