Astron. Astrophys. 359, 586-596 (2000)

The ¹²C/¹³C-ratio in cool carbon stars ^*

F.L. Schöier and H. Olofsson

Stockholm Observatory, 133 36 Saltsjöbaden, Sweden

Received 8 March 2000 / Accepted 8 May 2000

Abstract

We present observations of circumstellar millimetre-wave ¹³CO line emission towards a sample of 20 cool carbon stars. Using a detailed radiative transfer model we estimate the circumstellar ¹²CO/¹³CO-ratios, which we believe accurately measure the important stellar ¹²C/¹³C-ratios. For those optically bright carbon stars where it is possible, our derived ¹²C/¹³C-ratios are compared with the photospheric results, obtained with different methods. Our estimates agree well with those of Lambert et al. (1986).

It is shown that a straightforward determination of the ¹²CO/¹³CO-ratio from observed line intensity ratios is often hampered by optical depth effects, and that a detailed radiative transfer analysis is needed in order to determine reliable isotope ratios.

Key words: stars: abundances stars: AGB and post-AGB stars: carbon stars: circumstellar matter stars: late-type

* Presented in this paper is observational data collected using the Swedish-ESO submillimetre telescope, La Silla, Chile, the 20 m telescope at Onsala Space Observatory, Chalmers Tekniska Högskola, Sweden, and the NRAO 12 m telescope located at Kitt Peak, USA.

Send offprint requests to: F.L. Schöier (fredrik@astro.su.se)

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Contents

• 1. Introduction
• 2. The observations
• 3. Radiative transfer
• 4. Model results
  • 4.1. The CO modelling
  • 4.2. The CO modelling
  • 4.3. The circumstellar CO/CO-ratio
• 5. The ¹²C/¹³C-ratio
• 6. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000
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