PRE-MAIN-SEQUENCE LITHIUM BURNING - THE QUEST FOR A NEW STRUCTURAL PARAMETER

We present the results of stellar evolutionary computations to study t he sensitivity of lithium depletion in models of mass and metallicity close to solar, and its dependence on the micro - macro physical input s in the models, like thermodynamics, mixing, overshooting and the con vective model. We find that even marginal chemical inhomogeneities in stellar formation regions lead to a spread in Li-abundances for such s tars. We show that a general update in the physical inputs reverses th e previously established framework of the problem: solar models includ ing the most recent opacities and equation of state, a Full Spectrum o f Turbulence (FST) convection theory and a diffusive description of mi xing deplete lithium very efficiently during the pre-Main Sequence (pr e-MS). The present solar abundance could be then compatible with all t he initial lithium having been burnt during this phase, with no need t o invoke depletion during the MS lifetime. This new standard pre-MS Li -depletion is however not consistent with the observed Li-abundances v ersus mass in young open clusters (e.g. alpha Per and the Pleiades). W e are then led to interpret it as the maximum possible depletion for n on rotating, non magnetic stars, and look for physical mechanisms whic h can inhibit pre-MS depletion. We then included in our code an approx imated modeling of the influence of a magnetic field B on the convecti ve envelope. We show that even relatively low values of B are sufficie nt to largely inhibit Li-depletion during pre-MS. A dynamo generated m agnetic field of intensity related to stellar rotation would then lead to different Li-abundances at the end of the pre-MS phase, according to different rotational histories of stars, in qualitative agreement w ith the observed spread in Li-abundances in young open clusters stars. We discuss the possibility that the right balance between magnetic fi eld and metallicity (and overshooting, if any) can be the relevant par ameter in the evaluation of the amount of Li-7 which should eventually survive after the pre-MS phase. Models of masses 1.25 less than or eq ual to M/M. less than or equal to 0.7 are compared with the open clust er Li-7 - T-eff observations.