Lithium Isotopic Abundances in Metal‐poor Halo Stars

Abstract

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-LTE7Li 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 primordial7Li abundance is7Li/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 a6Li plateau at the level of log ε6Li ≈ 0.8; however, taking into account predictions for6Li destruction during the pre-main-sequence evolution tilts the plateau such that the6Li abundances apparently increase with metallicity. Our most noteworthy result is the detection of6Li in the very metal-poor star LP 815-43. Such a high6Li 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. Possible6Li production channels include protogalactic shocks and late-decaying or annihilating supersymmetric particles during the era of big bang nucleosynthesis. The presence of6Li limits the possible degree of stellar7Li depletion and thus sharpens the discrepancy with standard big bang nucleosynthesis.