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J.E. Norris (RSAA, ANU), S.G. Ryan (IoA, Cambridge Univ.), T.C. Beers (Michigan State Univ.)
We report lithium abundances (A(Li) = log(N(Li)/N(H)) + 12.00) obtained using the Anglo-Australian Telescope for 23 very metal-poor field main sequence stars, restricted to a very narrow range in effective temperature (6100 < T\rm eff < 6300 K) and metallicity ([Fe/H] < --2.5). Our aim is to accurately determine the dispersion of A(Li) at the lowest chemical abundances and earliest times, and hence to constrain both the amount of lithium which emerged from the Big Bang and possible later stellar processing and interstellar-medium creation.
The Li abundances are based on multiple, high-resolution (R = 40000), high signal-to-noise (~~100) spectra and are accurate to 0.033 dex. We find no dependence of A(Li) on effective temperature, but a clear one on [Fe/H] (dA(Li)/d[Fe/H] = 0.118 ± 0.023(1\sigma) dex/dex). Excluding two stars which are statistical non-members of the population, we find that the dispersion in A(Li) is 0.031 dex. That is, the Spite Plateau is ultra-thin. We argue that the primordial lithium value is given by A(Li)\rm p = 2.0, and discuss possible systematic errors (~~0.1 dex) and the cosmological implications of the result.
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