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Near infrared spectroscopy of M dwarfs. IV. A preliminary survey on the carbon isotopic ratios in M dwarfs

Based on the medium resolution near infrared spectra of 13CO (3,1) band, carbon isotopic ratios are estimated in 48 M dwarfs, for which we had determined the carbon and oxygen abundances from CO and H2O lines, respectively. We find clear evidence for the presence of a 13CO feature for the first time in the spectra of M dwarfs. The spectral resolution of our observed data, however, is not high enough to analyze the 13CO feature directly. Instead, we compare observed spectrum with synthetic spectra assuming 12C/13C = 10, 25, 50, 100, and 200 for each of 48 M dwarfs and estimate the best possible 12C/13C ratio by the chi-square analysis. The resulting 12C/13C ratios in M dwarfs distribute from 39 to a lower limit of 200. The mean value of 31 M dwarfs for which 12C/13C ratios are determined is 12C/13C = 87 +- 21 (p.e.), and that of 48 M dwarfs including those with the lower limit of 200 is 12C/13C > 127 +- 41 (p.e.). These results are somewhat larger than the 12C/13C ratio of the present interstellar matter (ISM) determined from the molecular lines observed in the millimeter and optical wavelength regions. Since the amount of 13C in the ISM has increased with time due to mass-loss from evolved stars, the 12C/13C ratios in M dwarfs, reflecting those of the past ISM, should be larger than those of the present ISM. In M dwarfs, log 13C/12C plotted against log Ac shows a large scatter without clear dependence on the metallicity. This result shows a marked contrast to log 16O/12C (= log Ao/Ac) plotted against log Ac, which shows a rather tight correlation with the larger value at the lower metallicity. Such a contrast can be a natural consequence that 16O and 12C are the primary products in the stellar nuclear synthesis while 13C is the secondary product, at least partly.