CS-22957-027 - A CARBON-RICH EXTREMELY-METAL-POOR STAR

We present a high-resolution spectroscopic analysis of the extremely-m etal-poor star CS 22957-027, which Beers et ai. (1992) noted to exhibi t a rather strong G-band. For T-eff = 4839 K, derived from broadband p hotometry, our analysis obtains log g = 2.25, and a very low metallici ty of [Fe/H]=-3.43 +/- 0.12. The carbon-to-iron ratio is found to be e nhanced by approximate to 2 dex relative to the solar value, similar t o the handful of other carbon-rich metal-deficient stars discussed by Norris et al. (1997a) and Barbuy et al. (1997). From the (CC)-C-12-C-1 3 (1,0)Swan bandhead and (CH)-C-13 lines the isotope ratio C-12/C-13 i s approximate to 10, indicating a C-13 enrichment. Nitrogen is also fo und to be enhanced by approximate to 1 dex. The s- process elements Sr and Ba are, surprisingly, found to be under-abundant relative to sola r, with [Sr/Fe]=-0.91 and [Ba/Fe]=-0.93. The star's low luminosity req uires that the chemical enrichment arises from mass transfer from an e volved companion rather than self-polluting dredge-up up processes. Ho wever, the s- process elements Sr and Ba are unusually low, with a sol ar ratio to one another, at variance with what found in the ''classica l'' CH stars. Finally we note that a feature due to (CH)-C-13 is prese nt in coincidence with the Th II 401.9129 nm resonance line. The blend is of relevance for the nucleo-chronology use of the Th II line in th ese extremely metal poor stars, when significant C-13 is present.