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Astron. Astrophys. 331, 1011-1021 (1998)

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9. Conclusions

We presented our new pre-MS solar models and discussed their sensitivity to some physical parameters like mass, magnetic field and metallicity, and to the input micro/macro physics.

A [FORMULA] decrease in the metallicity [FORMULA] adopted for the Sun, from [FORMULA], would bring the solar Li-abundance to the observed value. More generally, we may conclude that small inhomogeneties in the star formation regions might at least partially explain the large [FORMULA] abundance spread which is observed in low mass stars belonging to young open clusters.

We know that during the pre-MS phase stellar rotation produces non negligible magnetic fields via dynamo-effect. We therefore worked out a semi - quantitative description of how the picture presented changes due to the presence of a magnetic field [FORMULA], being aware of the fact that it contributes to inhibit convection, thus leading to higher residual Li-abundances. We show that a value of [FORMULA] G would provide in pre-MS all the Li-solar depletion (but no more), if we adopt the FST model to estimate the convective fluxes, a metallicity [FORMULA] and a diffusive treatment of turbulent mixing coupled to nuclear evolution. This latter description of mixing turns out to be mandatory if correct quantitative results are looked for. The usual MLT treatment of stellar convection is unable to reproduce the solar depletion, unless some extent of overshooting from the bottom of the convective envelope is invoked (of course, MS mechanisms of Li-depletion may act to bring the Sun to the present abundances.

A small amount of overshooting ([FORMULA]) would lead to very low values of [FORMULA], but this effect can be completely reset by a moderate magnetic field ([FORMULA] G). The right balance between a magnetic field acting in the direction of increasing Li-abundance in these stars, and a possible overshooting which favours Li-burning, could then also play a key-role in determining which is the amount of lithium surviving destruction after the pre-MS phase.

Accordingly, the Li-spread observed in open clusters stars can be mainly attributed to differences in their rotational rates, though observational uncertainties and small chemical inhomogeneties might contribute too. Playing with magnetic fields, we can construct models which nicely fit the observed [FORMULA] - [FORMULA] relation.

Before drawing definitive conclusions about the small value of the solar Li-abundance we must wait for a physically sound theory of the rotational history of the Sun during the pre-MS phase. Room is still left (even if not strictly required) for some amount of Li-depletion due to slow processes acting during the MS lifetime.

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© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998