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

CS 22957-027: a carbon-rich extremely-metal-poor star ^*

P. Bonifacio ¹, P. Molaro ¹, T.C. Beers ² and G. Vladilo <sup>1

1</sup> 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 = 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

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

Send offprint requests to: P. Bonifacio

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Abundance analysis
• 4. Abundance errors
  • 4.1. Atomic lines
  • 4.2. Molecular bands
• 5. Results
  • 5.1. Comparison with Norris et al.  (1997b)
• 6. Discussion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
helpdesk.link@springer.de