First stars VII - Lithium in extremely metal poor dwarfs ^*,**

¹ CIFIST Marie Curie Excellence Team
² Observatoire de Paris, GEPI, 92195 Meudon Cedex, France e-mail: [ Piercarlo.Bonifacio;Roger.Cayrel;Monique.Spite;Francois.Spite;Vanessa.Hill;
³ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy e-mail: molaro@ts.astro.it
⁴ Department of Physics & Astronomy and JINA: Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI 48824, USA e-mail: [thirupati;beers]@pa.msu.edu
⁵ GRAAL, Université de Montpellier II, 34095 Montpellier Cedex 05, France e-mail: Bertrand.Plez@graal.univ-montp2.fr
⁶ The Niels Bohr Institute, Astronomy, Juliane Maries Vej 30, 2100 Copenhagen, Denmark e-mail: [ja;birgitta]@astro.ku.dk
⁷ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain e-mail: ja@not.iac.es
⁸ Universidade de Sao Paulo, Departamento de Astronomia, Rua do Matao 1226, 05508-900 Sao Paulo, Brazil e-mail: barbuy@astro.iag.usp.br
⁹ European Southern Observatory, Casilla 19001, Santiago, Chile e-mail: edepagne@eso.org
¹⁰ European Southern Observatory (ESO), Karl-Schwarschild-Str. 2, 85749 Garching b. München, Germany e-mail: fprimas@eso.org

Received: 10 January 2006
Accepted: 2 October 2006

Abstract

Context.The primordial lithium abundance is a key prediction of models of big bang nucleosynthesis, and its abundance in metal-poor dwarfs (the Spite plateau) is an important, independent observational constraint on such models.

Aims.This study aims to determine the level and constancy of the Spite plateau as definitively as possible from homogeneous high-quality VLT-UVES spectra of 19 of the most metal-poor dwarf stars known.

Methods.Our high-resolution (), high spectra are analysed with OSMARCS 1D LTE model atmospheres and turbospectrum synthetic spectra to determine effective temperatures, surface gravities, and metallicities, as well as Li abundances for our stars.

Results.Eliminating a cool subgiant and a spectroscopic binary, we find 8 stars to have and 9 stars with . Our best value for the mean level of the plateau is . The scatter around the mean is entirely explained by our estimate of the observational error and does not allow for any intrinsic scatter in the Li abundances. In addition, we conclude that a systematic error of the order of 200 K in any of the current temperature scales remains possible. The iron excitation equilibria in our stars support our adopted temperature scale, which is based on a fit to wings of the Hα line, and disfavour hotter scales, which would lead to a higher Li abundance, but fail to achieve excitation equilibrium for iron.

Conclusions.We confirm the previously noted discrepancy between the Li abundance measured in extremely metal-poor turnoff stars and the primordial Li abundance predicted by standard Big-Bang nucleosynthesis models adopting the baryonic density inferred from WMAP. We discuss recent work explaining the discrepancy in terms of diffusion and find that uncertain temperature scales remain a major question.