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 C-12/C-13 ratios in 26 J-type carbon stars. The C-12/C-13 ratios are determined from lines of the CN red system around 8000 Angstrom, using the iso-intensity method and line -blanketed model atmospheres. The average of C-12/C-13 ratios in the 26 stars is 4.7 +/- 2.8 (standard de viation). All the stars studied, except for two stars have C-12/C-13 ratios smaller than 10. C-12/C-13 ratios as low as 1 similar to 2, which are lowe r than the value at the equilibrium of the CN-cycle, are found for a signif icant fraction of our sample, suggesting the operation of non-equilibrium n uclear processes. For several stars previously analyzed by other authors, o ur result shows fair agreement. The serious disagreement of C-12/C-13 ratio s, 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 C-12/C-13 ratios among the stars studied in the present work. This result implies that the m echanism responsible for low C-12/C-13 ratios in silicate carbon stars migh t be the same with that operating in other J-type carbon stars. In other wo rds, C-12/C-13 ratios in silicate carbon stars have turned out to give few clues to identify the mechanism responsible for their formation.