The 12C/13C Ratio in Carbon Stars

Abstract The carbon isotope ratio of 12C/13C is determined for nine carbon stars from an empirical pseudo curve-of-growth analysis on selected rotational lines of the CN red system. The results show the characteristic features as follows: (1) In the R-type carbon star HD 156074, the 12C/13C ratio is about 16; (2) in hydrogen-deficient carbon stars, the lower limits of the 12C/13C ratio are 500 and 100 for HD 137613 and HD 182040, respectively; (3) for J-type carbon stars in which 13C-features are known to be highly enhanced, the 12C/13C ratio turns out to be 3 and 6 for WZ Cas and RX Peg, respectively; and (4) for a majority of cool carbon stars (N-type), although 13C features are apparently very strong, the curve-of-growth effect should carefully be considered. Our empirical analysis, which is essentially an isointensities method, gives 12C/13C ratios of 20 (AQ And), 40 (Z Psc), 60 (ST Cas), and 100 (DS Peg). The above results imply that the 12C/13C ratio is abnormal only in peculiar carbon stars such as J-type stars (12C/13C ≲ 10) or in hydrogendeficient stars (12C/13C ≳ 100). On the other hand, the 12C/13C ratio is between 20 and 100 in a majority of cool carbon stars (N-type). In other words, 13C abundance relative to 12C is decreased in normal Ntype carbon stars as compared with K-M giant and supergiant stars in which 12C/13C ratios are generally lower than 20. This means that carbon enrichment in the evolution of a carbon star should have been produced by extensive mixing of 12C in a majority of cool carbon stars. Such a conclusion is consistent with the recent theory of mixing during thermal pulses due to instability at the He-shell burning stage.