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

7. Possible effect of mass accretion on the final Lithium abundance

All the above discussion ignores possible pre-MS mass accretion from the disk. Accretion occurring before the major Li-burning stage is not relevant in this context, but accretion during or following the burning phase may be relevant. Let us compare the accretion timescales with the evolutionary (thermal) pre-MS timescales (M, L and radius R in solar units). For the Sun, the main Li-depletion phase occurs at , while the radius is already close to the present one. Then , and accretion rates as large as /yr can affect the stellar and the surface lithium evolution; rates comparable to those observed in some classical TTauri's (e.g. Basri & Bertout 1989). Unfortunately, performing numerical computations with rotating models accreting from a disk is still far ahead of us; here we can only treat accretion as a first order perturbation -from the point of view of the surface chemistry- to hydrostatic, constant mass models. We have two cases:

• - accretion only during the Li-burning phase: the mass of the star will be larger at the end of burning, so it will show less lithium than that due for its final mass (remember how much Li-depletion increases when decreasing total mass). This possibility is interesting, but can not help us, as we are looking for mechanisms leading to less Li-depletion in pre-MS.
• - accretion also after the Li-burning phase. This case has been already discussed by D'Antona (1993). Starting from a 0.95 star and accreting 0.05 with interstellar Li-abundance, dilution will occur in the convective envelope which, at the end of Li-burning, is still . The final surface Li-abundance will be then about 1/4 of the interstellar one, consistent with those detected in some solar mass stars in open clusters.

Late accretion could be then, in principle, a way to increase the surface Li-abundance at the end of pre-MS. There is however an observational hint that this mechanism is not so efficient, at least in the majority of stars. The rotation rate attained by stars when they reach the MS depends on the variation of momentum of inertia during the pre-MS and on the exchange of momentum with the surroundings. It is commonly accepted (Königl 1991, Cameron & Campbell 1993) that the magnetic linkage between the star and the disk prevents the star's spin up which would be due to the decrease of the momentum of inertia in the approach to the main sequence. Stars preserving a disk (and accreting) for longer times will be then slower rotators on the MS (e.g. Bouvier 1994). Actually, observations indicate that faster rotators (having presumably lost their disks in earlier phases) display more lithium. The interplay among rotation, accretion and Li-depletion is far from being understood!

© European Southern Observatory (ESO) 1998

Online publication: March 3, 1998
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