Astron. Astrophys. 331, 1011-1021 (1998)
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 decrease in the metallicity
adopted for the Sun, from
, 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 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 , being
aware of the fact that it contributes to inhibit convection, thus
leading to higher residual Li-abundances. We show that a value of
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 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 ( ) would lead
to very low values of , but this effect can be
completely reset by a moderate magnetic field (
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 -
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.
© European Southern Observatory (ESO) 1998
Online publication: March 3, 1998
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