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 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) × 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 ≈ 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.