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Astron. Astrophys. 332, 672-680 (1998)

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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

1 Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34131 Trieste, Italy
2 Department of Physics and Astronomy, Michigan State University, East Lansing MI 48824, USA

Received 15 July 1997 / Accepted 17 December 1997

Abstract

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 [FORMULA] = 4839 K, derived from broadband photometry, our analysis obtains [FORMULA] = 2.25, and a very low metallicity of [Fe/H]= [FORMULA]. The carbon-to-iron ratio is found to be enhanced by [FORMULA] 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 [FORMULA] (1,0) Swan bandhead and [FORMULA] lines the isotope ratio [FORMULA] is [FORMULA] 10, indicating a [FORMULA] enrichment. Nitrogen is also found to be enhanced by [FORMULA] dex. The [FORMULA] 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 [FORMULA] 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 [FORMULA] is present.

Key words: stars:individual: CS 22957-027 – stars: abundances – stars: carbon – stars: Population II – Galaxy: halo

* This paper is based on observations collected at ESO, La Silla

Send offprint requests to: P. Bonifacio

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© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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