  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, non-LTE 7Li abundances from the LiI 670.8nm line
reveal a pronounced dependence on metallicity but with negligible scatter
around this trend. Very good agreement is found between the abundances from the
LiI 670.8nm line and the LiI 610.4nm line. The estimated primordial 7Li
abundance is $7Li/H = 1.1-1.5 x 10^-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, 6Li is
detected in nine of our 24 stars at the >2sigma significance level. Our
observations suggest the existence of a 6Li plateau at the level of log 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 LP815-43. Such a high 6Li
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 6Li production channels
include proto-galactic 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.
