Free access article

Issue A&A Volume 461, Number 1, January I 2007 Page(s) L13 - L16 Section Letters DOI 10.1051/0004-6361:20066552

A&A 461, L13-L16 (2007)
DOI: 10.1051/0004-6361:20066552

Letter

The extra-mixing efficiency in very low metallicity RGB stars

A. Recio-Blanco and P. de Laverny

Dpt. Cassiopée, Observatoire de la Côte d'Azur, CNRS/UMR 6202, BP 4229, 06304 Nice Cedex 4, France
    e-mail: [arecio;laverny]@obs-nice.fr

(Received 12 October 2006 / Accepted 6 November 2006)

Abstract
Aims.After the first dredge-up, low-mass Red Giant Branch (RGB) stars experience an extra-mixing episode that strongly affects the chemical abundances on their surface. This mixing occurs at the bump in the luminosity function. In this Letter we describe the efficiency of the extra-mixing in RGB stars found in very metal-poor globular clusters (GC).
Methods.The VLT/ISAAC spectra of twenty stars located between the bump and the tip of the RGB in four GCs with metallicities between [Fe/H]=-1.2 and -2.5 dex were collected. The carbon isotopic ratios on their surface were derived from the second overtone ( ) bands of the CO molecule at 2.3 m with the spectral synthesis method.
Results.It is found that the carbon isotopic ratios of very metal-poor GC stars always reach the equilibrium value of the CNO cycle almost immediately above the bump in the luminosity function. No additional mixing episode at brighter luminosities and no variations with the clusters' metallicity were detected. The extra-mixing is therefore found to be very efficient in metal-poor low-mass RGB stars, in very good agreement with theoretical expectations.

Key words: stars: abundances -- stars: evolution -- stars: late-type -- globular clusters: individual: general

© ESO 2006

What is OpenURL?