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Spectral analyses of three carbon-enhanced metal-poor stars

par Behara, Natalie Thérèse ;Bonifacio, Piercarlo;Ludwig, Hans Günter;Sbordone, Luca;Hernández, Jonay I. González;Caffau, Elisabetta
Référence Pos proceedings of science, (NIC X)068
Publication Publié, 2008

Article révisé par les pairs

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Résumé :

Approximately 20% of very metal-poor stars ([Fe/H] < -2.0) are strongly enhanced in carbon ([C/Fe] > +1.0). Such stars are referred to as carbon-enhanced metal-poor (CEMP) stars. A variety of abundance patterns are found among CEMP stars. Strong overabundances of nitrogen are common, and overabundances of neutron capture elements are often, however not always, present. The variety of abundance patterns among CEMP stars strongly suggests that this population of stars comprises several astrophysical origins. We are conducting a high-resolution follow-up of candidate EMP stars extracted from the Sloan Digital Sky Survey (SDSS; York et al. 2000) using UVES at the VLT. Three of the programme stars, SDSS J0912+0216, SDSS J1036+1212 and SDSS J1349-0229, where deliberately targetted as CEMP stars since a strong G band was evident from the SDSS spectra and the weakness of the Ca II K line testified their very low metallicity. The UVES high resolution follow-up confirmed the original findings ([Fe/H] < -2.50) and allowed a more detailed investigation of their chemical composition. We determined the carbon abundance from molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by classical 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. 3D carbon abundances were determined for all three stars, using CO5BOLD 3D hydrodynamical model atmospheres. 3D effects on the carbon abundance are found to be quite significant for these stars, with 3D corrections of up to -0.7 dex. Two of the stars, SDSS J0912+0216 and SDSS J1349-0229 exhibit an overabundance of neutron capture elements which classifies them as CEMP-s. Star SDSS J1036+1212, instead belongs to the elusive class of CEMP-no/s stars, with enhanced Ba, but deficient Sr, of which it is the third member discovered to date. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution- NonCommercial-ShareAlikeLicence