We have carried out a revised analysis and derivation of heavy-element (Z 35) abundances in the very metal-poor CH/CN-strong stars CS 22948-27 and CS 29497-34, originally discovered as part of the ongoing HK survey. These stars show among the highest C and N enhancements relative to Fe known, which, in combination with their relatively low effective temperatures, results in very strong CH, C2 and CN bands. The blending of lines, particularly in the blue, presents a great problem for abundance derivation from atomic lines based on observations with resolving power R . Nevertheless, by making use of spectrum synthesis fitting, it has been possible to derive the abundances of the neutron-capture heavy elements Sr, Y, Ba, La, Ce, Pr, Nd, Eu, and Dy.
The abundance patterns of the neutron-capture elements in CS 22948-27 and CS 29497-34 appear to be intermediate between s-process and a r-process enrichment patterns. These stars would therefore be different from the well-known "r-process star" CS 22892-52 (Sneden et al. 1996). The s-process contribution might be well due to a mass-transfer episode from an evolved companion star. Our radial velocity measurements show no significant variations, i.e., no indications of binarity, but are not sufficiently accurate, nor due they extend over a sufficiently long temporal baseline to exclude this hypothesis either.
Our present observations of these two stars suggest that these giants are not Pop III stars, polluted either by internal or external processes, but rather, they are very metal-poor stars, enhanced in C, N and s-process elements (and Eu) either because the matter which formed these stars happened to be enriched in these elements, or by mass transfer from an evolved companion. It is somewhat surprising that the two stars we have analyzed are so similar, even though they are separated by about in the sky, but of course no firm conclusions can be derived from such a small sample.
The full understanding of these very metal-poor and C,N-rich stars, revealed to be numerous in the HK survey, clearly warrants detailed investigation through the analysis of a much larger sample.
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999