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Astron. Astrophys. 322, L29-L32 (1997)

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3. Conclusion

This Letter analyses critically the quite classical claim that CS 22892-052 has been contaminated with a full solar mix of all the [FORMULA] r-nuclides. We conclude that this interpretation may be valid indeed, but is by far not the only possible one, as the observations in the limited [FORMULA] range are equally compatible with a mix that does not fit the solar one outside the observed domain. We assign this ambiguity to the fact that the observed CS 22892-052 pattern of abundances reflects primarily nuclear physics properties, and not one or another specificity of a blend of r-process events.

In such conditions, it is still not possible to make a final choice between the following two possibilities: (1) the available CS 22892-052 observations demonstrate that the r-process yields are almost perfectly solar irrespective of the mass and metallicity of the producing star (supernova). This invariance would imply that the relative r-nuclidic abundances are not influenced by the galactic chemical evolution effects, or (2) the whole [FORMULA] r-process abundance pattern is indeed affected by chemical evolution effects even if invariance in the [FORMULA] range is guaranteed by nuclear properties.

As a consequence of this ambiguity, the development of a cosmochronometry based on the Th content of CS 22892-052 remains highly insecure. In fact, case (2) would largely invalidate the conclusions drawn in this respect by Cowan et al. (1997). Obviously, a clarification of the present situation might come from further analyses of the r-process content of ultra-metal-poor stars, especially if abundances can be derived for the elements making the peaks in the solar r-process abundance distribution. As seen in Figs. 1 and 3, new observations, especially around Os, could help constraining the set of events that could have been responsible for the CS 22892-052 enrichment in r-nuclides.

In addition to the unresolved ambiguity defined above, the abundances measured in CS 22892-052 also exhibit some puzzling features. These are mainly the large Eu enrichment with respect to Fe and the C-rich nature of that star. This last property is not straightforwardly compatible with the contamination by an exploding massive star, which is generally considered as the privileged production site of the r-nuclides in spite of the difficulties of the present models (Takahashi and Janka, 1997)

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998