2007PASP..119..722S

This paper presents measurements of the ¹²C/¹³C abundance ratio among red giants in the globular cluster Messier 71. The stars observed have absolute magnitudes of M_V<1.0. The measurements are based on spectroscopy of 2.3 µm ^12,13CO bands using the NIRSPEC instrument on the Keck II Telescope. The resulting isotope ratios agree well with values derived by Briley et al. from CN bands at 8005 Å. All of the red giants in the NIRSPEC sample with M_V<0 are found to have ¹²C/¹³C < 10; an anticorrelation is found to exist between this ratio and λ4215 CN band strength, as well as a correlation with oxygen abundance. There are three giants in our sample in the magnitude range +1<M_V<0. These stars are found to have ¹²C/¹³C > 10, although the observational uncertainties are larger for such stars, due to both weaker CO bands and lower signal-to-noise ratios in the spectra. Thus, there may be some evidence that ¹²C/^ 13^C is decreasing with increasing stellar luminosity among the lower luminosity giants in our sample.