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 12C/13C ratio for five field giants with [Fe/H]=-0.6 including Arcturus and two stars in the globular cluster 47 Tuc. Using their very precise Mbol values obtained from HIPPARCOSparallaxes, we can place them into an evolutionary sequence. The sequence shows that the 12C/13C ratios ratios drops from 20 to near 7 between Mbol=+1 and +0.5, while Li disappears. At the same time the 12C/14N ratio diminishes by 0.2 to 0.4 dex. The two stars in 47 Tuc with Mbol near -2.0 show even lower 12C14N ratios by 0.4 dex indicating further mixing as they evolved to the top of the red giant branch. These observations confirm the existence of an extra-mixing process that becomes efficient on the red giant branch only when the low-mass stars reach the so-called luminosity function bump. We use the values of the carbon isotopic ratio observed in our sample to get constraints on the mu-barriers that may shield the central regions of a star from extra-mixing. We show that the same value of the critical gradient of molecular weight leads to 12C/13C ratios observed at different metallicities. This ``observational critical mu-gradient'' is in very good agreement with the one which is expected to stabilize meridional circulation. This result provides strong clues on the nature of the extra-mixing which occurs on the RGB, and indicates that it is related to rotation.