Astron. Astrophys. 332, 661-671 (1998)

On the determination of carbon isotopic ratios in cool carbon stars

P. de Laverny and B. Gustafsson

Astronomiska Observatoriet, Box 515, S-751 20 Uppsala, Sweden

Received 31 July 1997 / Accepted 30 October 1997

Abstract

We discuss the determination of carbon isotopic ratios  /   in N-type Galactic carbon stars. Ohnaka & Tsuji (1996) reported  /   ratios smaller by a factor 2 or 3 than the determinations of Lambert et al. (1986). Using synthetic spectra of carbon stars, we analyse systematic errors in their iso-intensity method. It is found to be rather sentitive to model parameters and blends. Furthermore, there are large discrepancies between the effective temperatures adopted by these two studies (mean discrepancy close to 260 K for the 20 stars in common). That, together with uncertainties due to model atmospheres, to carbon enrichments and effects of the blends, could explain most of the discrepancies between these two studies. The LGEH86 analysis is, on the other hand, rather insentitive to model parameters. We therefore conclude that large  /   ratios in cool Galactic carbon stars should be favoured.

Key words: stars: abundances stars: AGB and post-AGB stars: carbon molecular processes

Send offprint requests to: P. de Laverny (laverny@astro.uu.se)

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Contents

• 1. Introduction
• 2. Calculation of synthetic spectra
  • 2.1. Model atmospheres
  • 2.2. Line list
    • 2.2.1. Atomic data
    • 2.2.2. Molecular data
  • 2.3. Comparison with observations
    • 2.3.1. The Sun
    • 2.3.2. Cool carbon stars
• 3.  /   ratios from the iso-intensity method
• 4. Discussion
  • 4.1. Model atmosphere
  • 4.2. Determination of effective temperature
  • 4.3.  /   determinations of LGEH86
• 5. Conclusion
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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