We reconsider the role of pre-main sequence (pre-MS) Li depletion on the
basis of new observational and theoretical evidence: i) new observations of
Halpha emissions in young clusters show that mass accretion could be continuing
till the first stages of the MS, ii) theoretical implications from
helioseismology suggest large overshooting values below the bottom of the
convective envelopes. We argue here that a significant pre-MS 7Li destruction,
caused by efficient overshoot mixing, could be followed by a matter accretion
after 7Li depletion has ceased on MS thus restoring Li almost to the pristine
value. As a test case we show that a halo dwarf of 0.85 Msun with an extended
overshooting envelope starting with an initial abundance of A(Li) = 2.74 would
burn Li completely, but an accretion rate of the type 1e-8xe^{-t/3e6} Msun
yr−1 would restore Li to end with an A(Li) = 2.31. A self-regulating
process is required to produce similar final values in a range of different
stellar masses to explain the PopII Spite plateau. However, this framework
could explain why open cluster stars have lower Li abundances than the
pre-solar nebula, the absence of Li in the most metal poor dwarfs and a number
of other features which lack of a satisfactory explanation.Comment: To be published in Memorie della Societ\`a Astronomica Italiana
Supplementi Vol. 22, Proceedings of Lithium in the cosmos, Iocco F.,
Bonifacio P., Vangioni E., ed