1998A&A...332..204C

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 M_bol values obtained from HIPPARCOS parallaxes, we can place them into an evolutionary sequence. The sequence shows that the ¹²C/¹³C ratios drops from ∼20 to near 7 between M_bol=+1 and +0.5, while Li disappears. At the same time the ¹²C/¹⁴N ratio diminishes by 0.2 to 0.4dex. The two stars in 47 Tuc with Mbol near -2.0 show even lower ¹²C/¹⁴N ratios by 0.4dex 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 µ-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 µ-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.