How many low - mass stars do destroy 3 He ?

We recall the current status of the long-standing 3 He problem, and its possible connection with chemical anomalies on the red giant branch. In this context, we collect in the literature all the available observations of the carbon isotopic ratio in field and cluster giant stars. Using the HIPPARCOS paral-laxes, we get constraints on the evolutionary status of the field stars of the sample. This allows us to identify the stars that have passed the luminosity function bump and present 12 C/ 13 C ratios in disagreement with the standard predictions of stellar evolutionary models. We determine statistically what fraction of low mass stars experience an extra-mixing process on the red giant branch, and are then expected to destroy their 3 He at this evolutionary phase. The high number we get satisfies the galactic requirements for the evolution of the 3 He abundance. 1. The 3 He problem The evolution of 3 He in the Galaxy has first been considered to be straightforward, dominated by the net production of this light element by low mass stars (i.e., with masses lower than 2 M). In these objects, initial D is processed to 3 He during the pre-main sequence phase. Then, as described by Iben (1967), an 3 He peak builds up due to pp-reactions on the main sequence, and is engulfed in the stellar convective envelope during the first dredge-up on the lower red giant branch (RGB). Standard theory predicts that, once in the convective layers of the evolved star, 3 He cannot be destroyed because of the too cool temperature in these regions. It is finally ejected in the interstellar medium (ISM) in the latest stages of stellar evolution. In this standard view, the abundance of 3 He must increase in the Galaxy as soon as low-mass stars begin to pollute the ISM (Rood et al. 1976). One expects then to have constraints on the cosmological abundance of 3 He (Yang et al. 1984). Recent observations of a few planetary nebulae (PN; Rood et al. 1992, Balser et al. 1997) led to the measurement of 3 He in Send offprint requests to: [email protected] one object, NGC 3242 1. This PN, whose estimated initial mass is 1. for the most recent computations). This value however differs by almost two orders of magnitude with the 3 He abundance in the proto-solar nebula, (Balser et al. 1994). These low values are in clear …