We have determined the 12CO/13CO-ratio in the molecular envelopes around 20 carbon stars, using a detailed non-LTE radiative transfer code. The 12CO/13CO-ratios found range from 2.5 to 90, and we believe that these ratios accurately measure the stellar 12C/13C-ratios.
Due to optical depth effects, present mainly in the 12CO line, we find it necessary to treat in detail the radiative transfer in order to obtain reliable isotopomer ratios. For instance, for the two stars in common, we derive 12CO/13CO-ratios that are almost a factor of two higher than those of Greaves & Holland (1997) in their study of the evolution of the local interstellar 12C/13C-ratio.
An important ingredient in the model is the molecular abundance distribution in the CSE. We have here used the results of Mamon et al. (1988) for 12CO and 13CO, and we find that for both molecules these are consistent with the observations, although for 13CO we have only been able to test the results for a high mass loss rate object.
Our estimated 12C/13C-ratios agree well with those estimated in the photosphere by Lambert et al. (1986). Thus, we do not support the claims by other authors that the isotope ratios derived by Lambert et al. (1986) were too high, by about a factor of two.
© European Southern Observatory (ESO) 2000
Online publication: July 7, 2000