Meridional circulation and shear turbulencein low-mass RGB stars

Abstract

Red giant (RGB) stars in the field and in globular clusters presentabundance anomalies that can not be explained by standard stellar evolutionmodels. Some of these peculiarities, in particular those concerninglithium, carbon and nitrogen for stars more luminous than the bump, attestthe presence of extra-mixing processes at play inside the stars. Althoughtheir nature remains unclear, rotation has often been invoked as a possiblesource for mixing inside RGB stars (Sweigart & Mengel 1979;Charbonnel 1995; Denissenkov & Tout 2000). Much work hasbeen done during the last decade on the description of rotation-inducedmixing and on the transport of angular momentum and chemicals bymeridional circulation and shear turbulence (Zahn 1992, etc.). Within this framework, wepresent the first fully consistent computations of rotating low mass andlow metallicity stars from the Zero Age Main Sequence (ZAMS) to the upperRGB. When self-consistent evolution of the internal distribution of angularmomentum is taken into account, it is found that meridional circulation andshear instability by themselves are not able to produce the required amount of mixing to account for the observed abundance patterns.