Astron. Astrophys. 363, 605-616 (2000)

7. Conclusions

In this paper we have presented grids of intermediate mass models of metallicity appropriate for the LMC AGBs, Z=0.01, in order to reproduce the observed trend lithium vs luminosity, found by the survey of both Magellanic Clouds by Smith et al. (1995). We found that the interval of initial masses involved in lithium production is well defined within even considering all the uncertainties connected with the overshooting distance and the mass loss rate, and it is . More particularly, models with initial masses display a very peculiar behaviour, since they produce lithium even before the beginning of the first pulse.

Numerical simulations lead to the conclusion that large mass loss rates, approaching yr^-1, are required to fit the observations, otherwise we would expect to detect several large luminosity sources ( ) with negligible amount of lithium in their envelope, while the afore mentioned survey shows that practically all the AGB sources in the MCs with are lithium rich; if we rely on Blöcker's recipe for mass loss, we find that a value of the free parameter of is required in our models, while values can be disregarded since in these cases the most luminous Li-rich AGBs would have progenitors masses . These latter would produce lithium before they have TPs, so that they would not have s-process enriched envelopes, in contrast with the Smith et al. (1995) results.

We conclude that the most luminous Li-rich AGBs in the LMC represent the early AGB phases of the evolution of stars with initial masses . Our models of large progenitor mass () seem to be able to give a theoretical explanation of the existence in the LMC of AGB sources at and -7.6, which are long period, obscured variables (Wood et al 1992).

As a consequence of our calibration of mass loss, massive AGBs should not contribute significantly to the Lithium enrichment of the interstellar medium.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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