Abundance of lithium in unevolved stars and old disk stars : Interpretation and consequences.

High resolution spectra with high signal-to-noise ratios (typically 100) have been obtained for a few halo dwarfs with the 3^m.6 CFH telescope. These solar type stars are very old and extremely metal poor. The Li resonance line (λ6707) is observed, and is essentially produced by the ⁷Li isotope.

Although the mean metal abundance M/H varies from a factor t2 to 250 relative to the Sun, the abundance of lithium is very constant in all these stars, but the coolest ones where the convection zone is deep enough to induce ⁷Li destruction by proton fusion.

The non-destruction of ⁷Li in very old halo stars (of about the solar type) is to be attributed to the reduction of the size of the convective zone, reduction caused by the very small abundance of the heavy elements.

If the problem seems clear for the halo stars it is on the contrary very confusing in Population I stars. It seems however that in the old disk stars the destruction of lithium is also inhibited, probably also by a reduction of the depth of the convective zone due to the slight metal deficiency. It is pointed out, on the other hand, that among G type stars of Population I more than a half have a lithium abundance similar to that of the Hyades, although probably not half of them are as young as the Hyades.

It is suggested that the destruction of lithium in solar type stars is sometimes accelerated by the action of another process such as overshooting or turbulent diffusion, which is not always present.

We admit that the shape of the distribution of Li abundance in the Pleiades after Duncan (t98 t) rules out a significant destruction of lithium in the protostellar phase for solar type stars. Depletion of Lithium in halo stars by other processes is discussed and found unlikely. After discussion it is suggested that the abundance of lithium in halo dwarfs is therefore representative of the abundance of the interstellar matter which formed the stars and also that this matter itself retains the lithium abundances of the Big Bang, hardly altered.

We deduce a primordial value of the lithium abundance 11.2 10^-11 N_H (or, by mass X_Li = 5.96 10^-10).

The baryonic density of the Universe deduced from this value is compared to the densities deduced from the primordial abundance of deuterium and helium. All these densities are small, and this show that the Universe cannot be closed by nucleons.