6Li/7Li estimates for metal-poor stars

Context. The presence of the lithium-6 isotope in some metal-poor stars is a matter of surprise because of the high values observed. Non-standard models of Big Bang nucleosynthesis and pre-Galactic cosmic ray fusion and spallation have been proposed to explain these values. However, the observations of this light isotope are challenging which may make some detections disputable. Aims. The goal was to determine 6Li/7Li for a sample of metal-poor stars; three of them have been previously studied and the remaining two are new for this type of study. The purpose was to increase, if possible, the number of lithium-6 detections and to confirm previously published results. Methods. Spectra of the resonance doublet line of neutral lithium Li i 670.78 nm were taken with the high dispersion spectrograph at the Subaru 8.2 m-telescope for a sample of five metal-poor stars (−3.12 ≤ [Fe/H] ≤ −2.19). The contribution of lithium-6 to the total observed line profile was estimated from the 1D-LTE analysis of the line asymmetry. Results. Observed asymmetries could be reproduced assuming isotopic abundance ratios 6Li/7Li of the order of: 0.004 for BD +26◦ 3578, ∼0.010 for BD +02◦ 3375 and G 64-37, 0.025 for BD +20◦ 3603 and 0.047 for BD −04◦ 3208. We found that these results were very sensitive to several of the assumptions made in the analysis, in particular, the treatment of the residual structure in the analysed spectra. Our final estimates for the errors are respectively Δ6Li/7Li = ±0.028, 0.029, 0.039, 0.025 and 0.039. Conclusions. The 6Li/7Li ratios for the sample are comparable to or even lower than these error values, so that detections of lithium-6 can not safely be claimed despite of the high resolving power (R ∼ 95 000) and S/N (400−600).