Evolution of lithium in solar-type stars: clues from intermediate age clusters

We present Li abundances for 14 solar-type stars in the intermediate age (s imilar to 2 Gyr) clusters IC 4651 and NGC 3680. The log n(Li) vs. effective temperature distributions are com pared with those of the similar age clus ter NGC 752, of the younger Hyades (600 Myr) and of the older M 67 (4.5 Gyr ) and NGC 188 (6-7 Gyr) clusters. Neither IC 4651 nor NGC 3680 show the dis persion in Li which is observed in M 67. The 2 Gyr clusters have very simil ar Li vs. T-eff distributions; in addition, stars in the upper envelope of the M 67 distribution have the same Li content as stars in the 2 Gyr cluste rs, suggesting that either they have not suffered any significant depletion between similar to 2 and 4.5 Gyr or they had a much slower Li depletion. M echanisms that lead to Li depletion on the main sequence are discussed in t he light of these observations, None of the existing models seem to reprodu ce well the observed features.