Is 6Li in metal-poor halo stars produced in situ by solar-like flares?
Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, CNRS/IN2P3 and Université Paris-Sud, 91405 Orsay, France e-mail: email@example.com
Accepted: 15 March 2007
The high 6Li 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 6Li have been proposed in the literature. We study the possibility that the observed 6Li 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, 6Li could be mainly created in interactions of flare-accelerated 3He with stellar atmospheric 4He, via the reaction 4He(3He, p)6Li. Stellar dilution and destruction of flare-produced 6Li are evaluated from the evolutionary models of metal-poor stars developed by Richard and co-workers. Stellar depletion should be less important for 6Li atoms synthesized in flares than for those of protostellar origin. Theoretical frequency distributions of 6Li/7Li ratios are calculated using a Monte-Carlo method and compared with the observations. Excellent agreement is found with the measured 6Li/7Li distribution, when taking into account the contribution of protostellar 6Li originating from galactic cosmic-ray nucleosynthesis. We propose as an observational test of the model to seek for a positive correlation between 6Li/7Li and stellar rotation velocity. We also show that the amounts of 7Li, Be and B produced in flares of metal-poor halo stars are negligible as compared with the measured abundances of these species. 6Li in low-metallicity stars may be a unique evidence of the nuclear processes occuring in stellar flares.
Key words: stars: abundances / stars: flare / nuclear reactions, nucleosynthesis, abundances
© ESO, 2007