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Astron. Astrophys. 327, 342-348


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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`ohoku 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 MFORMULA 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


© European Southern Observatory (ESO) 1997

Online publication: October 1, 1997
Last change: April 8, 1998
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