High mass - loss carbon stars and the evolution of the local 12 C / 13 C ratio

The 12 C/ 13 C ratio has been measured in a sample of 10 carbon stars with high mass-loss rates, of ≥ 10 −5 M yr −1. This sample is complete for northern hemisphere carbon stars within ∼ 1 kpc of the Sun. Using the J=2–1 rotational lines of CO and 13 CO, the intensity ratio of 12 CO/ 13 CO is found to be 17 ± 11 for the sample, and the deduced 12 C/ 13 C abundance ratio, after corrections for CO optical depth effects, is 25 ± 13. The isotopic ratios are rather uniform (12–36 for 9 of the 10 stars), and there are no stars with very low 12 C/ 13 C values, such as the ratios of 3–4 found for a few low mass-loss carbon stars. The ejecta from the high mass-loss objects will dominate the evolution of the 12 C/ 13 C ratio 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 carbon. Thus the ejection of 13 C-rich material from AGB stars could explain the evolution of the local 12 C/ 13 C ratio from 89 at the time of the Sun's formation, to 60–70 now. From our data, it is estimated that the local ISM ratio should have evolved 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 13 C.