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Astron. Astrophys. 332, 204-214 (1998)

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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

1 Laboratoire d'Astrophysique de Toulouse, CNRS UMR 5572, Toulouse, France
2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
3 Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195-1580, USA
4 Program in Astronomy, Washington State University, Pullman, WA 99164-3113, USA
5 Guest observer, Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, Canada (Corinne.Charbonnel@obs-mip.fr; jbrown@delta.math.wsu.edu; wall@astro.washington.edu)

Received 9 September 1997 / Accepted 12 December 1997

Abstract

We have assembled accurate abundance data for Li, C, and N as well as the [FORMULA] ratio for five field giants with [Fe/H] [FORMULA] including Arcturus and two stars in the globular cluster 47 Tuc. Using their very precise [FORMULA] values obtained from HIPPARCOS parallaxes, we can place them into an evolutionary sequence. The sequence shows that the [FORMULA] ratios drops from [FORMULA] to near 7 between [FORMULA] = +1 and +0.5, while Li disappears. At the same time the [FORMULA] ratio diminishes by 0.2 to 0.4 dex. The two stars in 47 Tuc with Mbol near -2.0 show even lower [FORMULA] 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 [FORMULA] 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

* Based partially on observations obtained at the Dominion Astrophysical Observatory, National Research Council, Canada

Send offprint requests to: Corinne Charbonnel

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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