Carbon isotope ratio in carbon stars of the galactic halo

Abstract

We analysed CN red system (∼8000 Å) and C2 Swan system (∼4700 Å) 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. Results 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.