2007A&A...473L..37C

Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheric levels. Such asymmetries are usually neglected in abundance analyses. The determination of the ⁶Li/⁷Li isotopic ratio is prone to suffering from such asymmetries, as the contribution of ⁶Li is a slight blending reinforcement of the red wing of each component of the corresponding ⁷Li line, with respect to its blue wing. The present paper studies the halo star HD 74000 and estimates the impact of convection-related asymmetries on the Li isotopic ratio determination. Two methods are used to meet this aim. The first, which is purely empirical, consists in deriving a template profile from another element that can be assumed to originate in the same stellar atmospheric layers as LiI, producing absorption lines of approximately the same equivalent width as individual components of the ⁷LiI resonance line. The second method consists in conducting the abundance analysis based on NLTE line formation in a 3D hydrodynamical model atmosphere, taking into account the effects of photospheric convection. The results of the first method show that the convective asymmetry generates an excess absorption in the red wing of the ⁷Li absorption feature that mimics the presence of ⁶Li at a level comparable to the hitherto published values. This opens the possibility that only an upper limit on ⁶Li/⁷Li has thus far been derived. The second method confirms these findings. From this work, it appears that a systematic reappraisal of former determinations of ⁶Li abundances in halo stars is warranted.