Is 6-Li in metal-poor halo stars produced in situ by solar-like flares?

The high Li-6 abundances recently measured in metal-poor halo stars are far above the value predicted by Big Bang nucleosynthesis. They cannot be explained by galactic cosmic-ray interactions in the interstellarmedium either. Various pre-galactic sources of 6Li have been proposed in the literature. We study the possibility that the observed Li-6 was produced by repeated solar-like flares on the main sequence of these low-metallicity stars. The time-dependent flaring activity of these objects is estimated from the observed evolution of rotation-induced activity in Pop I dwarf stars. As in solar flares, Li-6 could be mainly created in interactions of flare-accelerated He-3 with stellar atmospheric He-4, via the reaction He-4(He-3, p)Li-6. Stellar dilution and destruction of flare- produced Li-6 are evaluated from the evolutionary models of metal-poor stars developed by Richard and co-workers. Stellar depletion should be less important for Li-6 atoms synthesized in flares than for those of protostellar origin. Theoretical frequency distributions of Li-6/Li-7 ratios are calculated using a Monte-Carlo method and compared with the observations. Excellent agreement is found with the measured Li-6/Li-7 distribution, when taking into account the contribution of protostellar Li-6 originating from galactic cosmic-ray nucleosynthesis. We propose as an observational test of the model to seek for a positive correlation between Li-6/Li-7 and stellar rotation velocity. We also show that the amounts of Li-7, Be and B produced in flares of metal-poor halo stars are negligible as compared with the measured abundances of these species. Li-6 in low- metallicity stars may be a unique evidence of the nuclear processes occuring in stellar flares.