Mixing processes during the evolution of red giants with moderate metal deficiencies: the role of molecular-weight barriers *
Corinne Charbonnel 1, 2,
Jeffery A. Brown 3, 4 and
George Wallerstein 3, 5
Received 9 September 1997 / Accepted 12 December 1997
We have assembled accurate abundance data for Li, C, and N as well as the ratio for five field giants with [Fe/H] including Arcturus and two stars in the globular cluster 47 Tuc. Using their very precise values obtained from HIPPARCOS parallaxes, we can place them into an evolutionary sequence. The sequence shows that the ratios drops from to near 7 between = +1 and +0.5, while Li disappears. At the same time the ratio diminishes by 0.2 to 0.4 dex. The two stars in 47 Tuc with Mbol near -2.0 show even lower 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 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 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.
Key words: stars: abundances stars: evolution stars: interiors stars: late-type
Send offprint requests to: Corinne Charbonnel
© European Southern Observatory (ESO) 1998
Online publication: March 10, 1998