1999A&A...343..923C

High signal to noise ratio spectra have been obtained with the GECKO spectrograph at CFHT, at a spectral resolution of 100 000, for three metal-poor stars in order to obtain more accurate abundances of the very fragile element ⁶Li. For two newly observed stars, BD +42 2667 and BD +36 2165 it appears that the first may have a detectable amount of ⁶Li, whereas no ⁶Li is found in the second one. The S/N ratio of only a few hundreds obtained for these two faint stars preclude however a firm conclusion. For the third star, the well known object HD 84937, a very high S/N of 650 per pixel (over 1000 per resolved spectral element) was obtained, yielding greatly improved accuracy over previous determinations. A value of ⁶Li/⁷Li = 0.052±0.019 (one sigma) is obtained. We also conclude that the no-⁶Li assumption is ruled out at the 95 per cent level, even in the most permissive case, when a variation of all the other free parameters (wavelength zero-point, continuum location, macroturbulent broadening, abundance of ⁷Li) is allowed. The possibility that the ⁶Li feature is an artifact due to a once suspected binarity of HD 84937 is discussed, with the conclusion that this assumption is ruled out by the extant data on the radial velocity of the object. The ⁶Li abundance is compared with recent models of formation of the light elements Li, Be and B. This comparison shows that ⁶Li is either undepleted, or only moderately depleted in HD 84937, from its initial value. Under the assumption that the atmospheric depletion of ⁶Li and ⁷Li in stars is by slow mixing with hot layers (underneath the convective zone), in which these elements can burn, we conclude that the depletion of ⁷Li by this mechanism in HD 84937 is less than 0.1dex. This new upper limit to the efficiency of the depletion of ⁷Li by slow mixing burning, in a star located on the Spite plateau, leads to a more secure estimation of the primordial abundance of ⁷Li. However, the effect of temperature inhomogeneities in the convective zone, on the derived abundance of lithium still remains to be accurately determined.