Lithium and mass loss in massive AGB stars in the Large Magellanic Cloud

The aim of this work is to use full evolutionary models to derive observati onal constraints on the mass loss rate of the upper Asymptotic Giant Branch (AGB) stars. The observations used to constrain the models are: i) the rel ative number of luminous Lithium rich AGBs in the Magellanic Clouds, with r espect to the total number of AGBs populating the luminosity range -6 great er than or equal to M-bol greater than or equal to -7; ii) the s-process en hancement of the same sample. The calibration of the mass loss rate we obta in gives feedbacks on the interpretation of observational data of obscured AGBs, and allows us to provide consistent lithium yields for these stars, t o be used to constrain the galactic chemical evolution. We find that: a) we can put lower and upper limits to the mass loss rate du ring the AGE phase; b) after a "visible" phase, the models evolve into a ph ase of strong mass loss, which can be identified with the obscured OH/IR st ars accessible only in the infrared; the models well reproduce the Period-M -bol loci of the obscured AGBs (Wood et al. 1992); c) the most massive AGBs (mass of progenitors, hereinafter M-ZAMS, similar to 6M.) are extremely lu minous (M-bol similar to -7.2 to -7.5); d) The lithium yield increases with the mass loss rate and with the total stellar mass, being maximum for AGE stars close to the lower limit for carbon semi-degenerate ignition. However , the mass loss calibration obtained in this work implies that massive AGBs do not contribute significantly to the lithium enrichment of the interstel lar medium.