X-shooter GTO: evidence for a population of extremely metal-poor, alpha-poor stars⋆
E. Caffau1,2⋆⋆, P. Bonifacio2, P. François3,2, L. Sbordone1,2, M. Spite2, L. Monaco4, B. Plez5, F. Spite2, S. Zaggia6, H.-G. Ludwig1,2, R. Cayrel2, P. Molaro7, S. Randich8, F. Hammer2 and V. Hill9
Zentrum für Astronomie der Universität Heidelberg,
Landessternwarte, Königstuhl 12,
2 GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, Place Jules Janssen, 92195 Meudon, France
3 UPJV, Université de Picardie Jules Verne, 33 Rue St Leu, 80080 Amiens, France
4 European Southern Observatory, Casilla 19001, 19, Santiago, Chile
5 Laboratoire Univers et Particules de Montpellier, LUPM, Université Montpellier 2, CNRS, 34095 Montpellier Cedex 5, France
6 Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova Vicolo dell’Osservatorio 5, 35122 Padova, Italy
7 Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
8 Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
9 Université de Nice Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, Laboratoire Cassiopée, BP 4229, 06304 Nice Cedex 4, France
Received: 5 July 2013
Accepted: 26 August 2013
Context. The extremely metal-poor stars are the direct descendants of the first generation stars. They carry the chemical signature of the pristine Universe at the time they formed, shortly after the Big Bang.
Aims. We aim to derive information about extremely metal-poor stars from their observed spectra.
Methods. Four extremely metal-poor stars were selected from the Sloan Digital Sky Survey (SDSS) and observed during the guaranteed observing time of X-shooter. The X-shooter spectra were analysed using an automatic code, MyGIsFOS, which is based on a traditional analysis method. It makes use of a synthetic grid computed from one-dimensional, plane-parallel, hydrostatic model atmospheres.
Results. The low metallicity derived from the SDSS spectra is confirmed here. Two kinds of stars are found. Two stars are confirmed to be extremely metal-poor, with no evidence of any enhancement in carbon. The two other stars are strongly enhanced in carbon. We could not derive iron abundance for one of them, while [Ca/H] is below –4.5. Two of the stars are members of the rare population of extremely metal-poor stars low in alpha elements.
Key words: stars: Population II / stars: abundances / Galaxy: abundances / Galaxy: formation / Galaxy: halo
© ESO, 2013