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Astron. Astrophys. 317, L63-L66 (1997)

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Letter to the Editor

Analysis of two CH/CN-strong very metal-poor stars *

B. Barbuy 1, R. Cayrel 2, M. Spite 3, T.C. Beers 4, F. Spite 3, B. Nordström 5 and P.E. Nissen 6

1 Universidade de São Paulo, CP 9638, 01065-970 São Paulo, Brazil
2 Observatoire de Paris, DASGAL, URA 335 du CNRS, 61 Av. de l'Observatoire, F-75014 Paris, France
3 Observatoire de Paris, Section de Meudon, DASGAL, URA 335 du CNRS, F-92195 Meudon Cedex, France
4 Michigan State University, Department of Physics and Astronomy, East Lansing, MI 48824, USA
5 Niels Bohr Institute of Astronomy, Physics & Geophysics, Copenhagen University, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark
6 Institute of Physics & Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark

Received 19 September 1996 / Accepted 8 October 1996

Abstract

Two very metal-deficient stars, BPS CS 22948-27 and BPS CS 29497-34, recently identified in the HK objective-prism/interference-filter survey of Beers and collaborators, which exhibit, at low resolution, strong CH and CN bands and metallicities [Fe/H] [FORMULA], have been observed at high spectral resolution and high S/N at the NTT at ESO with the EMMI echelle spectrograph. A preliminary analysis of these spectra shows that these objects are very unusual. The high-resolution work confirms the very low metallicity of these stars and indicates: (i) effective temperatures [FORMULA] [FORMULA] 4000 K and surface gravities log g [FORMULA] 0.0 to 1.5; (ii) a very large overabundance of carbon and nitrogen, by [FORMULA] 2 dex with respect to iron; (iii) a similar overabundance of a few neutron-capture elements, in particular Ba, La and Nd, also by [FORMULA] 2 dex; (iv) a low isotopic ratio [FORMULA] ; (v) a variable radial velocity for one of the stars (CS 22948-27). These findings suggest that the two stars are extreme cases of CH stars. The spectra are considerably richer in molecular bands than the spectrum of the remarkable star CS-22892-52, identified in the same survey and studied by Sneden et al. The determination of the ratio of s -process to r -process elements in these two stars must await observations in the blue spectral region.

Key words: Stars: abundances - Stars: Population II

* Based on observations collected at ESO, La Silla, Chile.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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