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

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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


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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© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000