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Astron. Astrophys. 317, 845-858 (1997)


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Carbon isotope ratio in carbon stars of the galactic halo

W. Aoki , 1, 2, T. Tsuji and 1

1 Institute of Astronomy, The University of Tokyo, 2-21-1, Osawa, Mitaka 181, Japan
2 Department of Astronomy, School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

Received 16 January 1996 / Accepted 5 May 1996

Abstract

We analysed CN red system ([FORMULA] 8000 Å) and C2 Swan system ([FORMULA] 4700 Å) to know carbon isotope ratios (12 C/13 C) for carbon stars in the Galactic halo, named CH stars. The isotope ratios are obtained for 6 CH stars by the curve-of-growth analysis of the isolated 12 CN and 13 CN lines. In this analysis, we compared directly 12 CN and 13 CN lines of similar intensities (iso-intensity method), and the resulting 12 C/13 C ratios are almost independent of the model atmosphere and its parameters. The 13 CN lines appear to be too weak in some CH stars, for which we applied the spectral synthesis method to the stronger C2 Swan band, obtained 12 C/13 C ratios for two stars and estimated the lower limits of 12 C/13 C ratios for two stars. In this case, however, the results depend on model atmosphere and its parameters. Results from our present and previous works show that most of them (12 stars) distribute around 12 C/13 C [FORMULA] 10 and two stars have very high values (12 C/13 C [FORMULA] 500). The distribution of 12 C/13 C ratios in CH stars is different from that of the population I carbon stars as well as population II oxygen-rich giants (G [FORMULA] K types). The CH stars of very high 12 C/13 C ratios can be explained by dredge-up of 12 C due to 3 [FORMULA] -process as in population I carbon stars (N-type). On the other hand the formation of the CH stars with low 12 C/13 C ratios requires the large supply of 12 C followed by a process of decreasing 12 C/13 C ratio.

Key words: stars: abundances – stars: carbon – stars: population II – stars: AGB and post AGB – Galaxy: halo


© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998

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