Forum Springer Astron. Astrophys.
Forum Whats New Search Orders

Astron. Astrophys. 355, 69-78 (2000)

Previous Section Next Section Title Page Table of Contents

8. Conclusions

We have observed a sample of 28 PNe in 12CO and 13CO. We have determined the isotopic ratio in 14 objects, 9 of which are new detections, and obtained robust upper limits in 4 other PNe. The observations reported in this paper improve the available data on the 12C/13C  ratio in PNe. The results confirm and extend earlier measurements which indicate values between 5 and 30, below those predicted by current AGB models. According to these calculations, the value of 12C/13C  in the ejecta of stars in the mass range between [FORMULA] and [FORMULA] [FORMULA] is of the order of 100. In order to agree with the observations, we suggest that stellar nucleosynthesis models should include the effects of extra-mixing to avoid the overproduction of 12C with respect to 13C not only during the RGB phase, but also in the latest phases of evolution. Our estimates of the mass of the progenitor stars support this conclusion, since all PNe considered here originated from stars with mass less than [FORMULA] [FORMULA]. The proposed extra-mixing mechanism is expected to be at work in this mass range (Hogan 1995, Charbonnel 1995) and represents at present the most likely solution to the so-called "3He problem".

Out of the six PNe with measured 3He abundances, only for one (NGC 6720) it was possible to estimate the 12C/13C  ratio. The remaining five objects show weak or no 12CO emission. In the case of NGC 3242 where the 3He measurements are the most reliable, the absence of CO emission in the envelope precludes a direct test of the predictions of stellar nucleosynthesis using millimeter line observations. We have suggested an alternative method to determine the isotopic abundance in this critical object.

We have used chemical evolution models to follow the variation with time and Galactocentric distance of the 3He abundance and 12C/13C  ratio. Whereas the model predictions for 3He are inconsistent with the observed abundance in the Solar System and in the interstellar medium unless extra-mixing is operative in at least [FORMULA] of low-mass stars, the evolution of 12C/13C  is remarkably insensitive to the effects of extra-mixing. The Galactic evolution of 12C and 13C is mainly governed by intermediate and massive stars, in which extra-mixing is not expected to occur.

Previous Section Next Section Title Page Table of Contents

© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000