The carbon isotope ratio of 120/lSC 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 liD 156074, the `2C/13C ratio is about 16; (2) in hydrogen-deficient carbon stars, the lower limits of the `2C/13C ratio are 500 and 100 for liD 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 `3C 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/ ) or in hydrogen- deficient stars (`2C/' ). On the other hand, the `2C/'3C ratio is between 20 and 100 in a majority of cool carbon stars (N-type). In other words, `3C abundance relative to 12C is decreased in normal N- type carbon stars as compared with K-M giant and supergiant stars in which 12C/'3C 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. Key words: Carbon stars; 12C/13C ratio; Empirical curve-of-growth analysis; Stellar evolution.
Publications of the Astronomical Society of Japan
- Pub Date:
- Carbon Stars:Chemical Composition;
- Carbon Stars:Evolution