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Astron. Astrophys. 345, 233-243 (1999)

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Quantitative analysis of carbon isotopic ratios in carbon stars

III. 26 J-type carbon stars including 5 silicate carbon stars

K. Ohnaka *  1,2 and T. Tsuji 1

1 Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
2 Technische Universität Berlin, Institut für Astronomie und Astrophysik, Hardenbergstraße 36, D-10623 Berlin, Germany

Received 4 January 1999 / Accepted 18 February 1999

Abstract

We present the result of a quantitative analysis of [FORMULA] ratios in 26 J-type carbon stars. The [FORMULA] ratios are determined from lines of the CN red system around 8000 Å, using the iso-intensity method and line-blanketed model atmospheres.

The average of [FORMULA] ratios in the 26 stars is [FORMULA] (standard deviation). All the stars studied, except for two stars, have [FORMULA] ratios smaller than 10. [FORMULA] ratios as low as [FORMULA], which are lower than the value at the equilibrium of the CN-cycle, are found for a significant fraction of our sample, suggesting the operation of non-equilibrium nuclear processes. For several stars previously analyzed by other authors, our result shows fair agreement. The serious disagreement of [FORMULA] ratios, which we reported for N-type carbon stars in our preceding paper, is not found for J-type carbon stars.

Five silicate carbon stars in our sample show no peculiar [FORMULA] ratios among the stars studied in the present work. This result implies that the mechanism responsible for low [FORMULA] ratios in silicate carbon stars might be the same with that operating in other J-type carbon stars. In other words, [FORMULA] ratios in silicate carbon stars have turned out to give few clues to identify the mechanism responsible for their formation.

Key words: stars: abundances – stars: carbon – stars: fundamental parameters – stars: AGB and post-AGB

* Research Fellow of the Japan Society for the Promotion of Science

Send offprint requests to: K. Ohnaka (Technische Universität Berlin), (ohnaka@export.physik.TU-Berlin.de)

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999
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