1997A&A...327..342G

The ¹²C/¹³C ratio has been measured in a sample of 10 carbon stars with high mass-loss rates, of ≥10^–5M_☉/yr. This sample is complete for northern hemisphere carbon stars within ∼1kpc of the Sun. Using the J=2-1 rotational lines of CO and ¹³CO, the intensity ratio of ¹²CO/¹³CO is found to be 17±11 for the sample, and the deduced ¹²C/¹³C 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 ¹²C/¹³C 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 ¹²C/¹³C ratio in the local interstellar medium, since mass return is dominated by AGB stars, and the stars of the highest {dot}(M) return the majority of the carbon. Thus the ejection of ¹³C-rich material from AGB stars could explain the evolution of the local ¹²C/¹³C 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 ¹³C.