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

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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 [FORMULA] 10-5 M [FORMULA] yr-1. This sample is complete for northern hemisphere carbon stars within [FORMULA] 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 [FORMULA] 11 for the sample, and the deduced 12C/13C abundance ratio, after corrections for CO optical depth effects, is 25 [FORMULA] 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 [FORMULA] 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 [FORMULA] 72 [FORMULA] 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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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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