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

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6. Conclusion

CS 22873-139 is a high velocity, very metal-deficient double-lined binary. Five high resoluton spectra at five different phases have been analyzed individually. The wings of the hydrogen lines indicate a rather low temperature, the colors seem to suggest a significantly hotter temperature, which would imply a smaller age. This problem has to be solved by more detailed observations. The abundances of the elements have been computed with the temperature indicated by the wings of the Hydrogen lines, but they have also been computed for the higher temperature indicated by the B-V color: the ratios of metal abundances are not significantly changed.

The large deficiency together with the lack of detection of Sr suggest that this star, born in a primitive phase of evolution, is perhaps a second generation star: the abundances in this star are therefore interesting.

The lithium line (veiled by the companion) is not measurable, but the upper limit of the lithium abundance may reach log[FORMULA](Li) [FORMULA] dex if the temperature is found from the hydrogen lines, and 2.3 dex if we rely on the photometry.

This very deficient star does not display the enhancement of the [FORMULA]-process elements, generally observed in the Pop II stars. The usual interpretations proposed in the literature for this non-enhancement of the [FORMULA]-elements do not seem to be adequate for stars which are both very deficient and very Sr-poor.

A possibility is a direct comparison of the abundance pattern with the predictions of the yields of individual models of zero-metallicity SN II. For some given masses of SN IIe, the models of Umeda et al. (2000) do not predict a significant enhancement of the [FORMULA]-process elements. Such models could fit the stars which do not show an enhancement of [FORMULA]-elements. In particular, a single SN II of 20 [FORMULA] could fit our star rather well, according to the model of Umeda et al., in spite of a discrepancy for Ti. Let us note that the definitive model atmosphere of the star could turn out to have a smaller gravity, which would imply a slightly lower abundance of Ti. As another possibility, Umeda et al. consider that the progenitor(s) of CS 22873-139 is(are) one(several) hypernova(e). The yields predicted for the SN IIe show significant variations from mass to mass and of different masses models could fit other Sr-poor stars, which show different abundance patterns.

It could then be that the abundances of each very deficient and very Sr-poor star reflect the products of a few (or even a single) SN II. Further analyses of extremely metal-deficient stars, are promising and should be rewarding.

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© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000