Lithium isotopic abundances in metal-poor stars

Abstract

We report on a survey of 7Li and 6Li isotopic abundances in metal-poor halo stars. The spectra of the 24 stars observed with VLT/UVES are of exceptionally high quality: S/N > 400 and resolving power R ≃ 120000. The 7Li abundances on our Hα Teff-scale show very small intrinsic scatter and a pronounced [Fe/H]-dependence. The resulting estimated primordial 7Li abundance is about 0.5 dex lower than predicted from Big Bang nucleosynthesis and the baryon density inferred by the cosmic microwave background. Nine of the stars yield a positive detection (>2σ) of 6Li, which suggests the existence of a 6Li plateau for halo stars. The most interesting result is the presence of 6Li in the very metal-poor ([Fe/H] = -2.74) dwarf LP815-43 at the level of 6Li/7Li ≃ 0.05 ±0.02. According to models for stellar 6Lidepletion due to diffusion or rotationally-induced mixing, a 0.5 dex 7Li depletion would require an unrealistic high initial 6Li abundance (log 6Li ≤ 2.0). Simultaneously, the observed high 6Li abundance at such low [Fe/H] can not be reconciled with existing models for Galactic cosmic ray spallation and α-fusion reactions. This opens up exciting prospects of pre-Galactic 6Li production, possibly due to cosmological cosmic rays or late-decaying massive particles such as the gravitino or neutralino in the Big Bang. © 2005 International Astronomical Union.