The lithium isotopic ratio in very metal-poor stars (Lind et al.)

Vol. 554
In section 8. Stellar atmospheres

The lithium isotopic ratio in very metal-poor stars

by K. Lind, J. Melendez, M. Asplund, R. Collet, and Z. Magic, A&A 554, A96

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Very metal-poor stars are ancient objects and thus the most important tracers of the cosmic abundance of lithium in the early universe. Big Bang nucleosynthesis predicts that they should show an undetectably low 6Li/7Li isotopic ratio. If this was not found to be the case, then an additional pregalactic production source of 6Li or a revision of the standard model of Big Bang nucleosynthesis would be required. It would also cast doubt on lithium depletion from stellar atmospheres as an explanation for the factor 3­5 discrepancy between the predicted primordial 7Li from the Big Bang and the observed value in metal-poor dwarf/turn-off stars. In this work, four Milky Way halo stars have been studied using improved model atmospheres and line formation treatment. For the first time, a combined 3D, nonlocal thermodynamic equilibrium (NLTE) modeling technique for Li, Na, and Ca lines is utilized to determine the lithium isotopic ratio. This allows superior modeling of the lithium resonance line and other neutral lines, leading to much more accurate determination of the 6Li/7Li isotopic ratio. Unlike previous less detailed modeling results from 1D and 3D LTE modeling, no star in this sample has any significant (2 sigma) detection of 6Li in NLTE. NLTE modeling can systematically reduce the best-fit lithium isotopic ratios by up to five percent. As a byproduct, the first ever 3D NLTE Ca and Na abundances of halo stars are presented, and they also reveal significant departures from LTE. In conclusion, the observational support for a significant and nonstandard 6Li production source in the early universe is substantially weakened by this new analysis, and thus it suggests that there is no need to revise standard Big Bang nucleosynthesis.