Abstract
The high 6 Li 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 interstellar medium either. Various pre-galactic sources of 6 Li have been proposed in the literature. We study the possibility that the observed 6 Li 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, 6 Li could be mainly created in interactions of flare-accelerated 3 He with stellar atmospheric 4 He, via the reaction 4 He( 3 He, p) 6 Li. Stellar dilution and destruction of flare-produced 6 Li are evaluated from the evolutionary models of metal-poor stars developed by Richard and co-workers. Stellar depletion should be less important for 6 Li atoms synthesized in flares than for those of protostellar origin. Theoretical frequency distributions of 6 Li/Li ratios are calculated using a Monte-Carlo method and compared with the observations. Excellent agreement is found with the measured 6 Li/Li distribution, when taking into account the contribution of protostellar 6 Li originating from galactic cosmic-ray nucleosynthesis. We propose as an observational test of the model to seek for a positive correlation between 6 Li/Li and stellar rotation velocity. We also show that the amounts of 7 Li, Be and B produced in flares of metal-poor halo stars are negligible as compared with the measured abundances of these species. 6 Li in low-metallicity stars may be a unique evidence of the nuclear processes occuring in stellar flares.
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