Convection and 6 Li in the atmospheres of metal-poor halo stars

Based on 3D hydrodynamical model atmospheres computed with the CO 5 BOLD code and 3D non-LTE (NLTE) line formation calculations, we study the effect of the convection-induced line asymmetry on the derived 6 Li abundance for a range in effective temperature, gravity, and metallicity covering the stars of the Asplund et al . (2006) sample. When the asymmetry effect is taken into account for this sample of stars, the resulting 6 Li/ 7 Li ratios are reduced by about 1.5% on average with respect to the isotopic ratios determined by Asplund et al . (2006). This purely theoretical correction diminishes the number of significant 6 Li detections from 9 to 4 (2σ criterion), or from 5 to 2 (3σ criterion). In view of this result the existence of a 6 Li plateau appears questionable. A careful reanalysis of individual objects by fitting the observed lithium 6707 A doublet both with 3D NLTE and 1D LTE synthetic line profiles confirms that the inferred 6 Li abundance is systematically lower when using 3D NLTE instead of 1D LTE line fitting. Nevertheless, halo stars with unquestionable 6 Li detection do exist even if analyzed in 3D-NLTE, the most prominent example being HD 84937.