MIXING PROCESSES DURING THE EVOLUTION OF RED GIANTS WITH MODERATE METAL DEFICIENCIES - THE ROLE OF MOLECULAR-WEIGHT BARRIERS

We have assembled accurate abundance data for Li, C, and N as well as the C-12/C-13 ratio for five field giants with [Fe/H] similar or equal to -0.6 including Arcturus and two stars in the globular cluster 47 T uc. Using their very precise M-bol values obtained from HIPPARCOS para llaxes, we can place them into an evolutionary sequence. The sequence shows that the C-12/C-13 ratios drops from similar to 20 to near 7 bet ween M-bol = +1 and +0.5, while Li disappears. At the same time the C- 12/N-14 ratio diminishes by 0.2 to 0.4 dex. The two stars in 47 Tuc wi th Mbol near -2.0 show even lower C-12/N-14 ratios by 0.4 dex indicati ng further mixing as they evolved to the top of the red giant branch. These observations confirm the existence of an extra-mixing process th at becomes efficient on the red giant branch only when the low-mass st ars reach the so-called luminosity function bump. We use the values of the carbon isotopic ratio observed in our sample to get constraints o n the mu-barriers that may shield the central regions of a star from e xtra-mixing. We show that the same value of the critical gradient of m olecular weight leads to C-12/C-13 ratios observed at different metall icities. This ''observational critical mu-gradient'' is in very good a greement with the one which is expected to stabilize meridional circul ation. This result provides strong clues on the nature of the extra-mi xing which occurs on the RGB, and indicates that it is related to rota tion.