1998A&A...332..672B

We present a high-resolution spectroscopic analysis of the extremely-metal-poor star CS 22957-027, which Beers et al. (1992AJ....103.1987B) noted to exhibit a rather strong G-band. For T_eff=4839K, derived from broadband photometry, our analysis obtains logg= 2.25, and a very low metallicity of [Fe/H]=-3.43±0.12 . The carbon-to-iron ratio is found to be enhanced by ≃2dex relative to the solar value, similar to the handful of other carbon-rich metal-deficient stars discussed by Norris et al. (1997ApJ...488..350N) and Barbuy et al. (1997A&A...317L..63B). From the ¹²C¹³C (1,0) Swan bandhead and ¹³CH lines the isotope ratio ¹²C/¹³C is ≃10, indicating a ¹³C enrichment. Nitrogen is also found to be enhanced by ≃1dex. The s- process elements Sr and Ba are, surprisingly, found to be under-abundant relative to solar, with [Sr/Fe]=-0.91 and [Ba/Fe]=-0.93. The star's low luminosity requires that the chemical enrichment arises from mass transfer from an evolved companion, rather than self-polluting dredge-up processes. However, the s- process elements Sr and Ba are unusually low, with a solar ratio to one another, at variance with what found in the ``classical'' CH stars. Finally we note that a feature due to ¹³CH is present 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 these extremely metal poor stars, when significant ¹³C is present.