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

ABSTRACT The T eff location of Pre-Main Sequence (PMS) evolutionary tracks depends on thetreatment ofover-adiabaticity.We present here the PMS evolutionarytrackscomputedby using the mixing length theory of convection (MLT) in which the α 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 withnon–grey ATLAS9 atmospheric boundary conditions and Full Spectrum of Turbulence(FST) convection model both in the atmosphere and in the interior. The comparisonof the new tracks with the location on the HR diagram of pre–MS binaries is notcompletely satisfactory, as some binary components are located at too low T eff . Be-sides, the pre–MS lithium depletion in the MLT–α 2D tracks is still much larger thanthat expected from the observations of lithium in young open clusters. This result issimilar to that of FST models. Thus, in spite of the fact that 2D RHD models shouldprovide a better convection description than any local model, their introduction is notsufficient to reconcile theory and observations. Lithium depletion in young clusterspoints towards a convection efficiency which, in pre–MS, should be smaller than inthe MS. The pre–MS lithium depletion decreases significantly in FST models if wereduce the solar metallicity down to the value suggested by Asplund et al. (2004), butthe corresponding solar model does not reproduce the depth of the convective zone asdetermined by helioseismology.Key words: stars: stellar structure; stars: convection; stars: abundances; stars: pre-main sequence