Lithium-6:: evolution from Big Bang to present

The primordial abundances of deuterium, He-4, and Li-7 are crucial to the determination of the baryon density of the Universe in the framework of standard Big Bang nucleosynthesis (BBN). Li-6 is only produced in tiny quantities and it is generally not considered to be a cosmological probe. However, recent major observational advances have produced an estimate of the Li-6/Li-7 ratio in a few very old stars in the galactic halo which impacts the question whether or not the lithium isotopes are depleted in the outer layers of halo stars, through proton induced reactions at the base of (or below) the convective zone. Here, we use (i) an empirical relation, independent of any evolutionary model, to set an upper limit on the Li-6 rise compatible with the very existence of the Spite's plateau (i.e. the flat lithium abundance measured in very old stars of the halo of our Galaxy of different iron content) and (ii) a well founded evolutionary model of light elements based on spallation production (Vangioni-Flam et al., 1997; Vangioni-Flam et al., 1998). Indeed, Li-6 is a pure product of spallation through the major production reactions, fast oxygen and alphas interacting on interstellar H, He (especially in the early Galaxy). The rapid nuclei are both synthesized and accelerated by SN II. In this context, the Li-6 evolution should go in step with that of beryllium and boron, recently observed by the Keck and HST telescopes. Li-6 adds a new constraint on the early spallation in the Galaxy. In particular, if confirmed, the Li-6/Be-9 ratio observed in two halo stars (HD 84937, BD + 26 degrees 3578 = HD 338529) gives strong boundary conditions on the composition and the spectrum of the rapid particles involved. Both methods converge to show that Li-6 is essentially intact in halo stars, and a fortiori Li-7, which is more tightly bound. Moreover, extrapolating empirical and theoretical evolutionary curves to the very low metallicities, we can define a range of the Li-6 abundance in the very early Galaxy consistent with Big Bang nucleosynthesis (5.6 x 10(-14) to 3 x 10(-13)). Following the evolution at increasing metallicity, we explain the abundance in the solar system within a factor of about 2. The whole evolution from Big Bang to present is reasonably reproduced, which demonstrates the general consistency of the present analysis of Li-6. The baryonic density derived from both lithium isotopes is between 1.5 to 3.5% of the critical one, in good agreement with the determination based on independent analyses. Consequently, thanks to these new data and theoretical developments, we show that Li-6 can be used to establish stellar Li-7 abundances as a valid tracer due to the fact that it allows to reinforce the Spite's plateau as a primordial test of BBN; on the other hand, its early evolution can be used to corroborate the calculated BBN abundances. In the framework of this work, a pregalactic alpha + alpha process producing Li-6 is not necessary. Finally, thanks to Li-6, the physics of spallative production of light elements should be more easily mastered when more data will become available. (C) 1999 Elsevier Science B.V. All rights reserved.