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Astron. Astrophys. 359, 586-596 (2000)

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1. Introduction

The 12C/13C-ratio is an important measure of stellar evolution and nucleosynthesis. Current theoretical models of stellar evolution on the asymptotic giant branch (AGB) predict that stars with "normal" chemical composition transform into carbon stars, from spectral type M via the intermediate states MS, S and SC, through the dredge-up of freshly synthesized carbon in He-shell flashes (Busso et al. 1999). This scenario is also confirmed by observations (Smith & Lambert 1985, 1990; Dominy et al. 1986; Lambert et al. 1986). Since mainly the 12C-isotope is synthesized, there should be a coeval evolution in the 12C/13C-ratio from the value determined during the first red giant evolution. In addition, processes like hot bottom burning will affect this ratio (Boothroyd et al. 1993), as well as set an upper mass limit for carbon stars. Thus, an accurate estimate of the 12C/13C-ratio should increase our understanding of the processes that lead to the formation of carbon stars (e.g., Forestini & Charbonnel 1997; Wallerstein & Knapp 1998). The 12C/13C-ratio is also an important tracer of the past starformation rate and stellar mass function (Prantzos et al. 1996; Greaves & Holland 1997).

In a classical paper, Lambert et al. (1986) determined the photospheric 12C/13C-ratios of 30 optically bright carbon stars, by fitting stellar atmosphere models to near-IR data on the isotopomers of C2, CO, and CN. A decade later Ohnaka & Tsuji (1996, 1999), based on a different method and data on the CN red system around 8000 Å, presented 12C/13C-ratios that, on the average, are about a factor of two lower than those of Lambert et al. (1986) for the same stars. The activity in this field was further increased with the published results of Abia & Isern (1997). They derived 12C/13C-ratios of 44 carbon stars, using the CN red system, which fell in between the results obtained by Lambert et al. (1986) and Ohnaka & Tsuji (1996). This is somewhat surprising since they used the same model atmospheres as Lambert et al. (1986), suggesting that the derived 12C/13C-ratios depend on the spectral features used. It is difficult to identify the main reason for the different results obtained by Lambert et al. (1986) and Ohnaka & Tsuji (1996, 1999). The former used high-resolution near-infrared data, while the latter used data obtained closer to optical wavelengths. Also the atmospheric models differ. de Laverny & Gustafsson (1998) have investigated the method used by Ohnaka & Tsuji (1996) and found that it is very sensitive to model parameters and blends. Their conclusion is that the larger 12C/13C-ratios determined by Lambert et al. (1986) are more reliable, since this analysis is rather insensitive to the adopted model parameters, and also the effect of blends is less severe, but the discussion continues (Ohnaka & Tsuji 1998; de Laverny & Gustafsson 1999). Recently, Ohnaka et al. (2000) have revised the values for three of the stars, which were originally published in Ohnaka & Tsuji (1996). Using new model atmospheres, they obtain 12C/13C-ratios that are larger by about 40%, i.e., closer to those estimated by Lambert et al. (1986).

To shed light on this disturbing controversy, we have performed independent estimates of the 12C/13C-ratio, using CO radio line emission from the circumstellar envelopes (CSEs), for a sample of carbon stars showing large discrepancies between the sets of photospheric estimates. Due to the weakness of the circumstellar 13CO lines, and the difficulties in the interpretation of the circumstellar emission, only a few attempts have been made to determine the 12CO/13CO ratio in the CSEs of carbon stars (e.g., Knapp & Chang 1985; Sopka et al. 1989; Greaves & Holland 1997). In this paper we present new observational results, as well as a detailed modelling of circumstellar 12CO and 13CO radio line emission.

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© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000