The lithium isotopic ratio in very metal-poor stars

Context. Un-evolved, very metal-poor stars are the most important tracers of the cosmic abundance of lithium in the early universe. Combining the standard Big Bang nucleosynthesis model with Galactic production through cosmic ray spallation, these stars at [Fe/H] < -2 are expected to show an undetectably small Li-6/Li-7 isotopic signature. Evidence to the contrary may necessitate an additional pre-galactic production source or a revision of the standard model of Big Bang nucleosynthesis. It would also cast doubts on Li depletion from stellar atmospheres as an explanation for the factor 3-5 discrepancy between the predicted primordial Li-7 from the Big Bang and the observed value in metal-poor dwarf/turn-off stars. Aims. We revisit the isotopic analysis of four halo stars, two with claimed Li-6-detections in the literature, to investigate the influence of improved model atmospheres and line formation treatment. Methods. For the first time, a combined 3D, non-local thermodynamic equilibrium ( NLTE) modelling technique for Li, Na, and Ca lines is utilised to constrain the intrinsic line-broadening and to determine the Li isotopic ratio. We discuss the influence of 3D NLTE effects on line profile shapes and assess the realism of our modelling using the Ca excitation and ionisation balance. Results. By accounting for NLTE line formation in realistic 3D hydrodynamical model atmospheres, we can model the Li resonance line and other neutral lines with a consistency that is superior to LTE, with no need for additional line asymmetry caused by the presence of Li-6. Contrary to the results from 1D and 3D LTE modelling, no star in our sample has a significant (2 sigma)detection of the lighter isotope in NLTE. Over a large parameter space, NLTE modelling systematically reduces the best-fit Li isotopic ratios by up to five percentage points. As a bi-product, we also present the first ever 3D NLTE Ca and Na abundances of halo stars, which reveal significant departures from LTE. Conclusions. The observational support for a significant and non-standard Li-6 production source in the early universe is substantially weakened by our findings.