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

1. Introduction

The recent observation of r-process elements ranging from Ba to Os, as well as Th, in the ultra-metal-poor halo star CS 22892-052 (Sneden et al., 1996) has brought some renewed excitement in the search of the origin of the heavy elements in the Universe. With a metallicity as low as [Fe/H]=-3.1 and a composition enriched in some pure r-elements, CS 22892-052 provides strong additional evidence that the production of heavy elements by the r-process already took place early in the history of the Galaxy. Moreover, the distribution of some 16 elements between Ba and Os shows a striking similarity with the solar system r-abundance distribution. It is tempting to conclude that this similarity reflects the absence of s-process contribution in this mass range, and that the solar system r-elements must originate from one or only a few astrophysical events, or, equivalently, that any astrophysical event producing r-elements gives rise to a solar system r-abundance distribution. This idea has always been present since Seeger et al. (1965) showed that, within the so-called canonical model, 3 astrophysical events (each of them characterized by one temperature, one neutron density and one irradiation time) are sufficient to explain the gross solar system r-abundance pattern. However, the conclusion that the solar system r-content originates from only a small number of astrophysical events corresponds to an extra step in the interpretation of the data for CS 22892-052. It should also be added that this star presents some puzzling peculiarities in addition to its solar distribution of some r-elements. First, it shows a remarkable overabundance of r-elements with respect to Fe ([Eu/Fe]=+1.7). Second, it is a C-rich star with and . This property is difficult to reconcile with the idea that the r-process develops during the supernova explosion of a massive metal-poor star. The corresponding ejecta is indeed predicted to exhibit a large overabundance of O with respect to C, in contradiction with the CS 22892-052 observations.

The aim of this Letter is to analyse in a critical way if indeed the r-process abundance data made available for CS 22892-052 imply that its whole r-process distribution is by necessity essentially solar. The answer to this question is important in order to constrain or unconstrain the nature of the r-process events that have contaminated the Galaxy in the early stages of its history. It is also of interest in order to evaluate the level of reliability of the attempts to build a Th cosmochronometry upon the CS 22892-052 abundance analysis. These attempts indeed rely heavily on the very basic assumption that the Th abundance inherited by that star with respect to the other elements is very close to solar.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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