ON THE SIGNIFICANCE OF POPULATION-II LI-6 ABUNDANCES

We study the significance of Li-6 abundances measured in metal-poor ha lo stars. We explore possible depletion factors for Li-6, defined as t he ratio of the protostellar to the observed abundance, in the three s tars where it has been detected; to this end, we assume that Li-6/H sc ales as O-16/H throughout the Galactic evolution. This assumption is m otivated by the recent observations of a similar scaling for Be-9 and by the fact that, apart from alpha - alpha fusion creation of Li-6, bo th elements are only created in p, alpha - C, N, O spallation reaction s. We examine possible uncertainties attached to the observations and to the modeling of Li-6 Galactic evolution; notably, we include a rece nt evaluation of the primordial production of Li-6. The depletion fact or D-6 in the hottest turnoff star HD 84937 is constrained to D-6 less than or equal to 4; this implies that at least one star on the Spite plateau has not depleted its primordial Li-7 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 Li-7 as a function of metallici ty and effective temperature, and yet allow D-7 greater than or equal to 2, while keeping D-6 less than or equal to 4. Therefore, Li-7 shoul d not be depleted by more than a factor similar or equal to 2 on the S pite plateau. If direct nuclear burning was the depletion mechanism, t hen Li-7 would be depleted by less than 2%. Moreover, all three Li-6 o bservations are in excellent agreement with all standard expectations: big bang nucleosynthesis with 2 < eta(10) < 6.5, and standard stellar isochrones of Li-6 survival in metal-poor stars. We also discuss poss ible deviations from our assumption of a scaling of Li-6/H with O-16/H because of alpha - alpha creation of Li-6 in various sites.