Astron. Astrophys. 327, 342-348 (1997)

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

J.S. Greaves and W.S. Holland

Joint Astronomy Centre, 660 N. A`hku Place, University Park, Hilo, Hawaii 96720, USA

Received 11 February 1997 / Accepted 12 June 1997

Abstract

The ¹²C/¹³C 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 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 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.

Key words: ISM: abundances - ISM: clouds

Send offprint requests to: J.S. Greaves

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Contents

• 1. Introduction
• 2. Observations
• 3. Results
  • 3.1. ¹²CO /¹³CO intensity ratios
  • 3.2. Line profiles
  • 3.3. LVG modelling
• 4. Discussion
  • 4.1. Evolution of ¹²C/¹³C in the ISM
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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