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 ^ { 7 } 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 ^ { 7 } Li abundance is ^ { 7 } { Li / H } = 1.1 - 1.5 \cdot 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 , ^ { 6 } Li is detected in nine of our 24 stars at the \geq 2 \sigma significance level .
Our observations suggest the existence of a ^ { 6 } Li plateau at the level of log \epsilon _ { { } ^ { 6 } Li } \approx 0.8 ; however , taking into account predictions for ^ { 6 } Li destruction during the pre-main sequence evolution tilts the plateau such that the ^ { 6 } Li abundances apparently increase with metallicity .
Our most noteworthy result is the detection of ^ { 6 } Li in the very metal-poor star LP 815-43 .
Such a high ^ { 6 } Li 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 ^ { 6 } Li production channels include proto-galactic shocks and late-decaying or annihilating supersymmetric particles during the era of Big Bang nucleosynthesis .
The presence of ^ { 6 } Li limits the possible degree of stellar ^ { 7 } Li depletion and thus sharpens the discrepancy with standard Big Bang nucleosynthesis .