SCYON Abstract

Received on February 21 2006

Rotational mixing in low-mass stars
II. Self-consistent models of Pop II RGB stars

AuthorsA. Palacios (1), C. Charbonnel (2,3), S. Talon (4) and L. Siess (1)
(1) Institut d'Astronomie et d'Astrophysique - Université Libre de Bruxelles, Bd du Triomphe, CP-226, B - 1050 Brussels, Belgium
(2) Observatoire de Genève, 51 Ch. des Maillettes, CH-1290 Sauverny, Switzerland
(3) Laboratoire d'Astrophysique de Toulouse-Tarbes, Observatoire Midi-Pyrénées, 14 av. E. Belin, F-31400 Toulouse , France
(4) Département de Physique, Université de Montréal, Montréal PQ H3C 3J7, Canada
Accepted byAstronomy & Astrophysics


In this paper we study the effects of rotation in low-mass, low-metallicity RGB stars. We present the first evolutionary models taking into account self-consistently the latest prescriptions for the transport of angular momentum by meridional circulation and shear turbulence in stellar interiors as well as the associated mixing processes for chemicals computed from the ZAMS to the upper RGB. We discuss in details the uncertainties associated with the physical description of the rotational mixing and study carefully their effects on the rotation profile, diffusion coefficients, structural evolution, lifetimes and chemical signatures at the stellar surface. We focus in particular on the various assumptions concerning the rotation law in the convective envelope, the initial rotation velocity distribution, the presence of μ-gradients and the treatment of the horizontal and vertical turbulence. This exploration leads to two main conclusions: (1) After the completion of the first dredge-up, the degree of differential rotation (and hence mixing) is maximised in the case of a differentially rotating convective envelope (i.e., jCE(r) = cst), as anticipated in previous studies. (2) Even with this assumption, and contrary to some previous claims, the present treatment for the evolution of the rotation profile and associated meridional circulation and shear turbulence does not lead to enough mixing of chemicals to explain the abundance anomalies in low-metallicity field and globular cluster RGB stars observed around the bump luminosity. This study raises questions that need to be addressed in a near future. These include for example the interaction between rotation and convection and the trigger of additional hydrodynamical instabilities.