Astron. Astrophys. 332, 721-731 (1998)

6. Summary and conclusions

We have presented a spectroscopic study of 13 bipolar PNe in which chemical information has been derived in different regions of the nebulae. While the present data do not allow to test for abundances variations through the nebulae smaller than, at best, some 10-20 for helium, 10-40 for oxygen, and larger for the other elements (see Sect. 4), they are still very useful in order to highlight large abundances variations as the ones claimed for other objects by some authors (Balick et al. 1994, Guerrero et al. 1995), as well as to derive accurate mean abundances for the objects, which can be obtained by averaging determinations obtained in regions of the objects with different ionization/excitation conditions.

Long-slit spectra obtained with larger class telescopes than the one used in the present study would be necessary in order to obtain high S/N in the very faint region of these nebulae, and minimize everywhere the errors due to the uncertainties in the flux measurements.

The results of this study are summarized below.

- Within the present errors, the He, O, and N abundances are constant throughout all the studied nebulae, with the possible exception of NGC 2440 where a tentative increase of a factor 1.4 of the N/O abundance ratio has been detected in the innermost region. Further work is however needed to confirm it.

- The Ne, Ar, and S abundances show systematic increases toward the outer regions of the nebulae. Although they are within the errors, these trends are systematic. They might be ascribed to inaccuracies in the icf s, as predicted by Alexander & Balick (1997). Note, however, that the corresponding apparent increase of the N abundance predicted by Alexander & Balick (1997) is generally not observed in our sample.

- The present sample contains some of the Galactic PNe with the highest He and N/O abundances known up to date. In particular, the highest He abundances cannot be reproduced by any current models of AGB evolution.

- The question of the possible Ne overabundance of the class of bipolar PNe, as compared to the general sample of Galactic PNe, remains controversial.

- A positive correlation is found between the observed ([NII]6548+6583)/H line ratio and the N/O abundance. Due to the large scatter in the correlation, its use is limited to statistical purposes, or as a tool to select objects with likely high N abundances in large spectroscopic or narrowband imaging databases.

- Oxygen depletion is suggested for the PNe with the highest N/O abundances, which is likely to be the consequence of efficient ON-cycle process in their high mass progenitors.

- High electron densities and temperatures are measured in the core of NGC 6537.

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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