Quantitative analysis of carbon isotopic ratios in carbon stars: III. 26 J-type carbon stars including 5 silicate carbon stars

We present the result of a quantitative analysis of ¹²C/ ¹³C ratios in 26 J-type carbon stars. The ¹²C/ ¹³C ratios are determined from lines of the CN red system around 8000 Å, using the iso-intensity method and line-blanketed model atmospheres. The average of ¹²C/ ¹³C ratios in the 26 stars is 4.7 ± 2.8 (standard deviation). All the stars studied, except for two stars, have ¹²C/ ¹³C ratios smaller than 10. ¹²C/ ¹³C ratios as low as 1 ∼ 2, which are lower than the value at the equilibrium of the CN-cycle, are found for a significant fraction of our sample, suggesting the operation of non-equilibrium nuclear processes. For several stars previously analyzed by other authors, our result shows fair agreement. The serious disagreement of ¹²C/ ¹³C ratios, which we reported for N-type carbon stars in our preceding paper, is not found for J-type carbon stars. Five silicate carbon stars in our sample show no peculiar ¹²C/ ¹³C ratios among the stars studied in the present work. This result implies that the mechanism responsible for low ¹²C/ ¹³C ratios in silicate carbon stars might be the same with that operating in other J-type carbon stars. In other words, ¹²C/ ¹³C ratios in silicate carbon stars have turned out to give few clues to identify the mechanism responsible for their formation.