Evidence for non-standard mixing on the red giant branch

Abstract

Based on the new grid of standard stellar models computed at solar metallicity by the Geneva group (Schaller et al. 1992) with the recent OPAL opacity tables (Rogers & Iglesias 1992), we follow the theoretical evolution of the ¹²C/¹³C ratio along the red giant branch of low and intermediate mass stars. We compare the evolutionary predictions with observations of the surface isotopic ratios in evolved stars of galactic clusters. This leads us to conclude that the theoretical and observational first dredgeup perfectly agree, but that in low mass stars (M. ≤ 2M_⊙) an extra-mixing occurs later on the giant branch which produces an additional decrease of the surface ¹²C/¹³C ratio. We show that this extra-mixing is only efficient when the hydrogen-burning shell reaches the chemical discontinuity created by the convective envelope at its maximum extent. We speculate on the nature of the extra-mixing process.