Lithium isotopic abundances in metal-poor halo stars

Very high quality spectra of 24 metal-poor halo dwarfs and subgiants have been acquired with ESO's VLT/UVES for the purpose of determining Li isotopic abundances. The derived one-dimensional, non-LTE Li-7 abundances from the Li I 670.8 nm line reveal a pronounced dependence on metallicity but with negligible scatter around this trend. Very good agreement is found between the abundances from the Li I 670.8 nm line and the Li I 610.4 nm line. The estimated primordial Li-7 abundance is Li-7/H (1.1-1.5) x 10(-10), which is a factor of 3-4 lower than predicted from standard big bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, Li-6 is detected in 9 of our 24 stars at the >= 2 sigma significance level. Our observations suggest the existence of a Li-6 plateau at the level of log epsilon(6Li) approximate to 0.8; however, taking into account predictions for Li-6 destruction during the pre-main-sequence evolution tilts the plateau such that the Li-6 abundances apparently increase with metallicity. Our most noteworthy result is the detection of Li-6 in the very metal-poor star LP 815-43. Such a high Li-6 abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic-ray spallation and alpha-fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible Li-6 production channels include protogalactic shocks and late-decaying or annihilating supersymmetric particles during the era of big bang nucleosynthesis. The presence of Li-6 limits the possible degree of stellar Li-7 depletion and thus sharpens the discrepancy with standard big bang nucleosynthesis.