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

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1. Introduction

During the pre-MS phase, stellar temperatures rise to values at which light elements burning begins to occur. Lithium has been the subject of extensive work during the recent past (Bodenheimer 1965; D'Antona & Mazzitelli 1984; Vandenberg & Poll 1989; Proffitt & Michaud 1989; Deliyannis et al. 1990; Deliyannis & Pinsonneault 1990; Ryan et al. 1992; Swenson et al. 1994; D'Antona & Mazzitelli 1994) both due to its cosmological interest, and because it is regarded as a good indicator of the pre-MS history of stars. [FORMULA] is destroyed via the reaction [FORMULA] starting at [FORMULA], first at the centre of the star and then, if the temperature is large enough, also at the base of the convective envelope. This extablishes a tight relationship between the inward extension of the envelope and the amount of [FORMULA] left at the surface. We expect larger depletion in lower mass stars which, being cooler, have deeper envelopes and, more generally, in the presence of physical processes -like overshooting- which mix the surface matter with deeper and hotter layers.

Historically theoretical research on Li-depletion is connected with the following, still unsolved, problems:

i) the large observed solar Li-depletion, as compared to other solar-type stars.

ii) the spread in Li-abundance observed at the end of the pre-MS in young open clusters, at least for masses [FORMULA].

Point (i) deserves particular attention, since no theoretical model until now was able to reproduce the large drop in the solar [FORMULA] abundance, from the initial value of [FORMULA] found in the solar system (Michaud & Charbonneau 1991) to the current value of [FORMULA] (Anders & Grevesse 1989). Other depletion mechanisms have been then suggested, acting during the MS lifetime of the Sun. Examples are: rotationally induced mixing associated with the transport of angular momentum through the radiative - convective interface (Endal & Sofia 1976; Pinsonneault et al. 1990), complicated by the interaction between meridional circulation and turbulence (Zahn 1992; Chaboyer & Zahn 1992); gravitational settling (Michaud & Charbonneau 1991); both the above mechanisms (Chaboyer et al. 1995a,b); mixing by gravity waves (García Lopez & Spruit 1991; Schatzman 1993). A recent attempt to explain Li-observations for the Hyades through pre-MS depletion alone is by Swenson et al. (1994), who included in their models the Rogers & Iglesias (1992a) opacities, larger than former ones around the Li-burning temperatures. More pre-MS depletions was found than with previous models; the huge solar Li-depletion could not, however, be achieved (Faulkner 1991; Swenson & Faulkner 1992).

D'Antona & Mazzitelli (1994, hereinafter DM94) have shown that a key role for the inward penetration of the surface convective envelope is played by the turbulent convection model. Contrarily to the mixing lenght theory (MLT) based models, which predict relatively low Li-depletion, models employing the recent Canuto & Mazzitelli (1991, 1992) full spectrum of turbulence (FST) convection model displayed a quite large Li-depletion (by a factor [FORMULA]). This was not consistent with the observations of open clusters (Martin 1997); in fact, the comparisons made in DM94 clearly showed that only FST models computed with the Kurucz (1993) low-T opacities, or MLT models with Alexander & Ferguson (1994) low-T opacities could be made consistent with the [FORMULA] Per data. Examining these results, Strom (1994) suggested that the Li-depletions in DM94 should be considered as the upper boundaries to pre-MS depletion.

More recently, several improvements in the input physics became available, and it is now worth to update the computations. The bottom line of the present work is that the new pre-MS "standard" solar models show such a huge Li-depletion that the solar [FORMULA] abundance can be achieved by pre-MS depletion only, without invoking any long-term MS mechanism. This result is at variance with our whole previous understanding of the observational patterns. In particular, even considering the result correct for the Sun itself, it is not possible that all stars of solar mass have the same large depletion in pre-MS, otherwhise one can not explain lithium in young open clusters.

We then examine how Li-depletion depends on some physical and chemical inputs. In the end we introduce a new parameter and study -still in a first approximation- the influence of a magnetic field B on the thermal structures of the largely convective pre-MS stars (Spruit 1987). The results show that Li-depletion markedly decreases when increasing B ; it can be orders of magnitude lower even with fields which, when emerging at the surface, are already quite low ([FORMULA] G). As a strict correlation between the intensity of a dynamo-generated magnetic field and rotation is expected to exist, these findings qualitatively explain the correlation rotation - [FORMULA] which is found among open cluster stars and, in a first order approximation, also the quantitative, observed Li-abundances. This is seen when we finally show some comparisons of our results with observations of lithium in the clusters [FORMULA] Per, Pleiades and Praesepe.

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© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998