1996A&A...310..933O

We present a result of quantitative analysis of ¹²C/¹³C ratios in 62 N-type and 15 SC-type carbon stars. By the use of CCD as a detector we can obtain spectra of resolution ∼20,000 with enough signal-to-noise ratios for a large number of carbon stars, for which ¹²C/¹³C ratios have not yet been derived. Carbon isotopic ratios are determined from lines of the CN red system around 8000A, based on the iso-intensity method and line-blanketed model atmospheres. The average of ¹²C/¹³C ratios in 62 N-type carbon stars is found to be 27±11 (standard deviation). The majority of the N-type carbon stars studied (about 85%) are found to have ¹²C/¹³C ratios less than 40, and the number of stars which have ¹²C/¹³C ratios larger than 40 is found to be relatively small. This result shows a marked contrast to some of the previous results that have shown the opposite distribution, namely, ¹²C/¹³C ratios mostly larger than 40 in N-type carbon stars. The average of ¹²C/¹³C ratios in 15 SC-type carbon stars is found to be 22±14 (standard deviation). Most of the SC-type carbon stars studied are found to have ¹²C/¹³C ratios larger than 10, while only three of them turn out to be ¹³C-rich. This is in contrast with the earlier classification based on low resolution spectra which classified them as J-type, that is, ¹³C-rich. The earlier temperature scale which classified SC-type carbon stars as the latest (C8-9) based on their strong NaI D lines can not be necessarily justified. The strong NaI D lines of SC stars should be attributed to the peculiar atmospheric structure due to C/O ratios very near to unity. The resulting ¹²C/¹³C ratios are partly consistent with the scenario in which M giants evolve through SC-type to N-type carbon stars, as ¹²C produced during the helium shell flash is added to the envelope.