First stars XI. Chemical composition of the extremely metal-poor dwarfs in the binary CS 22876-032

¹ CIFIST Marie Curie Excellence Team
² GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France e-mail: [Jonay.Gonzalez-Hernandez;Piercarlo.Bonifacio;Hans.Ludwig;Elisabetta.Caffau;Roger.Cayrel;Monique.Spite;Francois.Spite;Vanessa.Hill;Patrick.Francois]@obspm.fr
³ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy e-mail: molaro@ts.astro.it
⁴ GRAAL, Université de Montpellier II, 34095 Montpellier Cedex 05, France e-mail: Bertrand.Plez@graal.univ-montp2.fr
⁵ Department of Physics and Astronomy, CSCE: Center for the Study of Cosmic Evolution, and JINA: Joint Institute for Nuclear Astrophysics, Michigan State University, E. Lansing, MI 48824, USA e-mail: [thirupati;beers]@pa.msu.edu
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 Las Cumbres Observatory, Goleta, CA 93117, USA e-mail: edepagne@lcogt.net
¹⁰ European Southern Observatory (ESO), Karl-Schwarschild-Str. 2, 85749 Garching b. München, Germany e-mail: fprimas@eso.org

Received: 12 October 2007
Accepted: 6 December 2007

Abstract

Context.Unevolved metal-poor stars constitute a fossil record of the early Galaxy, and can provide invaluable information on the properties of the first generations of stars. Binary systems also provide direct information on the stellar masses of their member stars.

Aims.The purpose of this investigation is a detailed abundance study of the double-lined spectroscopic binary CS 22876-032, which comprises the two most metal-poor dwarfs known.

Methods.We used high-resolution, high-S/N ratio spectra from the UVES spectrograph at the ESO VLT telescope. Long-term radial-velocity measurements and broad-band photometry allowed us to determine improved orbital elements and stellar parameters for both components. We used OSMARCS 1D models and the turbospectrum spectral synthesis code to determine the abundances of Li, O, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Co and Ni. We also used the CO⁵BOLD model atmosphere code to compute the 3D abundance corrections, notably for Li and O.

Results.We find a metallicity of [Fe/H] ~ -3.6 for both stars, using 1D models with 3D corrections of ~-0.1 dex from averaged 3D models. We determine the oxygen abundance from the near-UV OH bands; the 3D corrections are large, -1 and -1.5 dex for the secondary and primary respectively, and yield [O/Fe] ~ 0.8, close to the high-quality results obtained from the [OI] 630 nm line in metal-poor giants. Other [ α/Fe] ratios are consistent with those measured in other dwarfs and giants with similar [Fe/H], although Ca and Si are somewhat low ([X/Fe] 0). Other element ratios follow those of other halo stars. The Li abundance of the primary star is consistent with the Spite plateau, but the secondary shows a lower abundance; 3D corrections are small.

Conclusions.The Li abundance in the primary star supports the extension of the Spite Plateau value at the lowest metallicities, without any decrease. The low abundance in the secondary star could be explained by endogenic Li depletion, due to its cooler temperature. If this is not the case, another, yet unknown mechanism may be causing increased scatter in A(Li) at the lowest metallicities.