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The 12C/13C-ratio in cool carbon stars *
F.L. Schöier and
Received 8 March 2000 / Accepted 8 May 2000
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 (email@example.com)
Online publication: July 7, 2000