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
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], the abundance ratios of CS 22873-139 are not similar to those of a typical Population II star. In particular, the -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] . 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
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© European Southern Observatory (ESO) 2000
Online publication: August 23, 2000