ON THE DETERMINATION OF CARBON ISOTOPIC-RATIOS IN COOL CARBON STARS

We discuss the determination of carbon isotopic ratios C-12/C-13 in N- type Galactic carbon stars. Ohnaka & Tsuji (1996) reported C-12/C-13 r atios Smaller by a factor 2 or 3 than the determinations of Lambert et al. (1986). Using synthetic spectra of carbon stars, we analyse syste matic errors in their iso-intensity method. It is found to be rather s entitive to model parameters and blends. Furthermore, there are large discrepancies between the effective temperatures adopted by these two studies (mean discrepancy close to 260 K for the 20 stars in common). That, together with uncertainties due to model atmospheres, to carbon enrichments and effects of the blends, could explain most of the discr epancies between these two studies. The LGEH86 analysis is, on the oth er hand, rather insentitive to model parameters. We therefore conclude that large C-12/C-13 ratios in cool Galactic carbon stars should be f avoured.