HIGH MASS-LOSS CARBON STARS AND THE EVOLUTION OF THE LOCAL C-12 C-13 RATIO/

The C-12/C-13 ratio has been measured in a sample of 10 carbon stars w ith high mass-loss rates, of greater than or equal to 10(-5) M-. yr(-1 ). This sample is complete for northern hemisphere carbon stars within similar to 1 kpc of the Sun. Using the J=2-1 rotational lines of CO a nd (CO)-C-13, the intensity ratio of (CO)-C-12/(CO)-C-13 is found to b e 17 +/- 11 for the sample, and the deduced C-12/C-13 abundance ratio, after corrections for CO optical depth effects, is 25 +/- 13. The iso topic ratios are rather uniform (12-36 for 9 of the 10 stars), and the re are no stars with very low C-12/C-13 values, such as the ratios of 3-4 found for a few low mass-loss carbon stars. The ejecta from the hi gh mass-loss objects will dominate the evolution of the C-12/C-13 rati o in the local interstellar medium, since mass return is dominated by AGB stars, and the stars of the highest M return the majority of the c arbon. Thus the ejection of C-13-rich material from AGB stars could ex plain the evolution of the local C-12/C-13 ratio from 89 at the time o f the Sun's formation, to 60-70 now. From our data, it is estimated th at the local ISM ratio should have evolved to approximate to 72 +/- 8 at the present time, which agrees with ISM observations. The AGB stars can therefore fully explain the enrichment of the local ISM in C-13.