On the Significance of Population II6Li Abundances

We study the significance of 6Li abundances measured in metal-poor halo stars. We explore possible depletion factors for 6Li, defined as the ratio of the protostellar to the observed abundance, in the three stars where it has been detected; to this end, we assume that 6Li/H scales as 16O/H throughout the Galactic evolution. This assumption is motivated by the recent observations of a similar scaling for 9Be and by the fact that, apart from α - α fusion creation of 6Li, both elements are only created in p, α - C, N, O spallation reactions. We examine possible uncertainties attached to the observations and to the modeling of 6Li Galactic evolution; notably, we include a recent evaluation of the primordial production of 6Li. The depletion factor D6 in the hottest turnoff star HD 84937 is constrained to D6 ≤ 4; this implies that at least one star on the Spite plateau has not depleted its primordial 7Li by more than a factor 4, even by extreme dilution. This constraint is in fact stronger when one takes all constraints into account; indeed, no current stellar model is able to reproduce the abundances of 7Li as a function of metallicity and effective temperature, and yet allow D7 ≥ 2, while keeping D6 ≤ 4. Therefore,7Li should not be depleted by more than a factor ≃2 on the Spite plateau. If direct nuclear burning was the depletion mechanism, then 7Li would be depleted by less than 2%. Moreover, all three 6Li observations are in excellent agreement with all standard expectations: big bang nucleosynthesis with 2 < η10 < 6.5, and standard stellar isochrones of 6Li survival in metal-poor stars. We also discuss possible deviations from our assumption of a scaling of 6Li/H with 16O/H because of α - α creation of 6Li in various sites.