Tese de Doutorado #080: Luiz Mendes

We have investigated the combined effects of rotation and internal angular momentum redistribution on the structure and evolution of low mass stars, from the pre-main sequence to the main sequence phase. As a tool for that study, the ATON stellar evolutionary code (Mazzitelli 1989; Ventura et al. 1998) has been modified in order to include those effects. Rotation was implemented according to the equipotential technique developed by Kippenhahn and Thomas (1970) and later improved by Endal and Sofia (1976). Angular momentum redistribution in radiative regions was modeled through an advection-diffusion partial differential equation based on the framework originally introduced by Chaboyer and Zahn (1992), which is based on the sole assumption of stronger turbulent transport in the horizontal direction than in the vertical one. The diffusion coefficient of this equation is obtained from characteristic lengths and velocities of typical rotation-induced hydrodynamical instabilities. This improved code was used to compute a series of rotating low mass stellar models (with masses ranging from 1.2 Msun down to 0.6 Msun). Regarding the structural (hydrostatic) effects of rotation, the general features of these models show that rotating stars behave as if they were non-rotating stars of slightly lower masses, in accordance with previous results by other researchers. A study of this mass-lowering effect for the considered range of masses shows that rotation decreases lithium depletion while the star is fully convective but increases it as soon as the star develops a radiative core. The net effect is a enhanced lithium depletion, in disagreement with observational data which suggest that faster rotators in young open clusters experience less lithium depletion. Angular momentum redistribution in the considered models is very effective in smoothing their internal angular velocity profile as soon as the star reaches the zero age main sequence, but fails to reproduce the flat solar rotation rate obtained from helioseismology, indicating that, in the Sun, angular momentum transport is more efficient than current models. The internal angular momentum transport also contributes to a still higher lithium depletion than the models computed with only the structural effects of rotation, thus suggesting that other physical phenomena must play a role regarding both lithium depletion and the rotation profile evolution of these stars.