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CS 22957-027: a carbon-rich extremely-metal-poor star *
P. Bonifacio 1,
P. Molaro 1,
T.C. Beers 2 and
G. Vladilo 1
Received 15 July 1997 / Accepted 17 December 1997
We present a high-resolution spectroscopic analysis of the extremely-metal-poor star CS 22957-027, which Beers et al. (1992) noted to exhibit a rather strong G-band. For = 4839 K, derived from broadband photometry, our analysis obtains = 2.25, and a very low metallicity of [Fe/H]= . The carbon-to-iron ratio is found to be enhanced by dex relative to the solar value, similar to the handful of other carbon-rich metal-deficient stars discussed by Norris et al. (1997a) and Barbuy et al. (1997). From the (1,0) Swan bandhead and lines the isotope ratio is 10, indicating a enrichment. Nitrogen is also found to be enhanced by dex. The 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 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 13 CH 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 is present.
Key words: stars:individual: CS 22957-027 stars: abundances stars: carbon stars: Population II Galaxy: halo
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© European Southern Observatory (ESO) 1998
Online publication: March 23, 1998