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: tatische@csnsm.in2p3.fr

Received: 25 October 2006
Accepted: 15 March 2007

Abstract

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.