LITHIUM ISOTOPIC ABUNDANCES IN METAL-POOR HALO STARS

摘要

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 ~7Li 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 ~7Li abundance is ~7Li/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, ~6Li is detected in 9 of our 24 stars at the ≥ 2 σ significance level. Our observations suggest the existence of a ~6Li plateau at the level of log ε_(~6Li) ≈ 0.8; however, taking into account predictions for ~6Li destruction during the pre-main-sequence evolution tilts the plateau such that the ~6Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of ~6Li in the very metal-poor star LP 815—43. Such a high ~6Li abundance during these early Galactic epochs is very difficult to achieve by Galactic cosmic-ray spallation and α-fusion reactions. It is concluded that both Li isotopes have a pre-Galactic origin. Possible ~6Li production channels include protogalactic shocks and late-decaying or annihilating supersymmetric particles during the era of big bang nucleosynthesis. The presence of ~6Li limits the possible degree of stellar ~7Li depletion and thus sharpens the discrepancy with standard big bang nucleosynthesis.