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Astron. Astrophys. 360, 1077-1085 (2000)

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Element abundance ratios in an extremely metal-poor binary star: CS 22873-139 *

M. Spite 1, E. Depagne 1, B. Nordström 2, V. Hill 3, R. Cayrel 1, F. Spite 1 and T.C. Beers 4

1 Observatoire de Paris-Meudon, DASGAL, URM 8633 du CNRS, 92125 Meudon Cedex, France (Monique.Spite,Francois.Spite,Roger.Cayrel@obspm.fr)
2 Niels Bohr Institute for Astronomy, Physics and Geophysics, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen, Denmark (birgitta@astro.ku.dk)
3 European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany (vhill@eso.org)
4 Michigan State University, Department of Physics and Astronomy, East Lansing, MI 48824, USA (beers@pa.msu.edu)

Received 13 April 2000 / Accepted 13 June 2000


High-resolution spectra of the extremely metal-poor double-lined spectroscopic binary star BPS CS 22873-139 have been analyzed to determine the metallicity and abundance ratios for a number of elements. From the analysis of the collected radial velocities, new orbital elements are derived as well as an improved mass ratio. In spite of its extreme metal deficiency, [Fe/H][FORMULA], the abundance ratios of CS 22873-139 are not similar to those of a typical Population II star. In particular, the [FORMULA]-elements are not enhanced relative to iron, and the strontium abundance is very low.

The abundance pattern of CS 22873-139 is compared to the patterns exhibited by other metal-poor stars which exhibit also very low strontium abundances. There is a large spread of elemental abundance ratios among these stars, suggesting that low strontium abundance may be associated with a variety of nucleosynthesis histories. The abundance ratios of CS 22873-139 are, surprisingly, very similar to those found in a common proper-motion pair HD 134439, HD 134440, even though the metallicity of this system is almost two dex higher, [Fe/H] [FORMULA]. The ratios are compared to those of another very metal-poor binary star BPS CS 22876-032. The unusual abundance pattern of CS 22873-139 is discussed (by comparison to the predicted yields of zero-metal SN II and hypernovae).

The lithium doublet at 6707 Å is not detected in CS 22873-139, but an abundance of lithium consistent with the Spite plateau cannot be excluded, based on the present data.

Key words: stars: abundances – stars: binaries: spectroscopic – stars: Population II – stars: individual: BPS CS 22873-139 – nuclear reactions, nucleosynthesis, abundances

* Based on observations made at the European Southern Observatory, La Silla, Chile

Send offprint requests to: M. Spite

© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000