2000A&A...355...69P

We present the results of a study aimed at determining the ¹²C/¹³C ratio in two samples of planetary nebulae (PNe) by means of millimeter wave observations of ¹²CO and ¹³CO. The first group includes six PNe which have been observed in the ³He⁺ hyperfine transition by Balser et al. (1997ApJ...483..320B); the other group consists of 22 nebulae with rich molecular envelopes. We have determined the carbon isotopic ratio in 14 objects, 9 of which are new detections. The results indicate a range of values of ¹²C/¹³C between 9 and 23. We estimate the mass of the progenitors of the PNe of our sample and combine this information with the derived ¹²C/¹³C isotopic ratios to test the predictions of stellar nucleosynthesis models. We find that the majority of PNe have isotopic ratios below the values expected from current standard asymptotic giant branch models in the mass range of interest. We suggest that the progenitors of the PNe must have undergone a non-standard mixing process during their red giant phase and/or asymptotic giant phase, resulting in a significant enhancement of the ¹³C abundance in the surface layers. Our study confirms a similar behaviour inferred from spectroscopic observations of field population II stars and globular cluster giants, and extends it to the final stages of stellar evolution. Finally, we discuss the implications of our results on models of Galactic chemical evolution of ³He and ¹²C/¹³C.