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 1D, nonLTE Li 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 abundance is Li/H = 1.1 − 1.5 · 10, which is a factor of three to four lower than predicted from standard Big Bang nucleosynthesis with the baryon density inferred from the cosmic microwave background. Interestingly, Li is detected in nine of our 24 stars at the ≥ 2σ significance level. Our observations suggest the existence of a Li plateau at the level of log ǫ6Li ≈ 0.8; however, taking into account predictions for Li destruction during the pre-main sequence evolution tilts the plateau such that the Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of Li in the very metal-poor star LP815-43. Such a high Li 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 Li production channels include proto-galactic shocks and late-decaying or annihilating supersymmetric particles during the era of Big Bang nucleosynthesis. The presence of Li limits the possible degree of stellar Li depletion and thus sharpens the discrepancy with standard Big Bang nucleosynthesis. Subject headings: Stars: abundances – Stars: atmospheres – Stars: Population II – Galaxy: evolution – Cosmology: early Universe