New light on the old problem of lithium pre--MS depletion: models with 2D RHD convection

Abstract

The Teff location of Pre-Main Sequence (PMS) evolutionary tracks depends on the treatment of over-adiabaticity. We present here the PMS evolutionary tracks computed by using the mixing length theory of convection (MLT) in which the alpha_MLT=l/H_p parameter calibration is based on 2D--hydrodynamical models (Ludwig et al. 1999). These MLT--α^2D stellar models and tracks are very similar to those computed with non--grey ATLAS9 atmospheric boundary conditions and Full Spectrum of Turbulence (FST) convection model both in the atmosphere and in the interior. The comparison of the new tracks with the location on the HR diagram of pre--MS binariesis not completely satisfactory, as some binary components are located at too low \teff. Besides, the pre--MS lithium depletion in the MLT--α^2D tracks is still much larger than that expected from the observations of lithium in young open clusters. This result is similar to that of FST models. Thus, in spite of the fact that 2D RHD models should provide a better convection description than any local model, their introduction is not sufficient to reconcile theory and observations. Lithium depletion in young clusters points towards a convection efficiency which, in pre--MS, should be smaller than in the MS. The pre--MS lithium depletion decreases significantly in FST models if we reduce the solar metallicity down to the value suggested by Asplund et al.(2004), but the corresponding solar model does not reproduce the depth of the convective zone as determined by helioseismology.