1997A&A...317..845A

We analysed CN red system (∼8000Å) and C₂ Swan system (∼4700Å) to know carbon isotope ratios (¹²C/¹³C) for carbon stars in the Galactic halo, named CH stars. The isotope ratios are obtained for 6 CH stars by the curve-of-growth analysis of the isolated ¹²CN and ¹³CN lines. In this analysis, we compared directly ¹²CN and ¹³CN lines of similar intensities (iso-intensity method), and the resulting ¹²C/¹³C ratios are almost independent of the model atmosphere and its parameters. The ¹³CN lines appear to be too weak in some CH stars, for which we applied the spectral synthesis method to the stronger C₂ Swan band, obtained ¹²C/¹³C ratios for two stars and estimated the lower limits of ¹²C/¹³C ratios for two stars. In this case, however, the results depend on model atmosphere and its parameters.
Result. from our present and previous works show that most of them (12 stars) distribute around ¹²C/¹³C∼10 and two stars have very high values (¹²C/¹³C≥500). The distribution of ¹²C/¹³C ratios in CH stars is different from that of the population I carbon stars as well as population II oxygen-rich giants (G∼K types). The CH stars of very high ¹²C/¹³C ratios can be explained by dredge-up of ¹²C due to 3α-process as in population I carbon stars (N-type). On the other hand the formation of the CH stars with low ¹²C/¹³C ratios requires the large supply of ¹²C followed by a process of decreasing ¹²C/¹³C ratio.