Rotation and Internal Gravity Waves in Low-Mass Stars

Abstract

We discuss the successes and failures of self-consistent rotating models of main sequence and slightly evolved low-mass stars. We focus in particular on the strongest observational constraints which are the lithium surface abundances and the solar rotation profile deduced from helioseismology. We recall that the hot side of the so-called lithium dip is well explained by hydrodynamical models where the transport of angular momentum is carried out by meridional circulation and shear turbulence. For cooler stars however the transport of angular momentum is dominated by another process. We show that internal gravity waves are the best candidate and we explain how the mass dependence of this mechanism is expected to resolve the enigma of the lithium dip in terms of rotational mixing, forming a coherent picture in main sequence stars of all masses. 1 Rotating Models and the Blue Side of the Li Dip In many locations in the Hertzsprung-Russell diagram, stars exhibit signatures of processes that require challenging modeling beyond the standard stellar theory. In this context, rotation has become a major ingredient of modern stellar models, especially when abundance anomalies have to be accounted for. In the most sophisticated theoretical developments (i.e., Zahn 1992, Maeder 1995, Talon & Zahn 1997, Maeder & Zahn 1998, Palacios et al. 2003), the internal rotation law evolves as a result of contraction, expansion, mass loss, meridional circulation (hereafter MC) and shear turbulence, and the mixing of the chemicals is directly linked to the rotation profile. Such a self consistent treatment has been successfully applied in various parts of the HR diagram over a large range of stellar masses and evolutionary stages (see the review by Maeder & Meynet 2000, and the papers by Maeder, Meynet and Palacios in this volume). One of the most striking signatures of transport processes in low-mass stars is the so-called Li dip (Fig. 1). This drop-off in the Li content of MS F-stars in a range of 300K in Teff centered around 6700K was first discovered in the Hyades by Wallerstein et al. (1965) and latter confirmed by Boesgaard & Tripicco (1986). This feature appears in all galactic clusters older than ∼ 200 Myrs as well as in field stars (Balachandran 1995). Boesgaard (1987) noticed that at the Teff of Li dip in the Hyades also occurs a sharp drop in rotational velocities (Fig. 1). Rotation was then suggested to play a dominant role in the build up of the Li dip.