Carbon isotope ratios on the upper red giant branch of Messier 71

This paper presents measurements of the C-12/C-13 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 mu m (CO)-C-12, 13 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 angstrom. All of the red giants in the NIRSPEC sample with are found to have C-12/C-13 < 10; an anticorrelation is found to exist between this ratio and lambda 4215 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. Tese stars are found to have C-12/C-13 < 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 C-12/C-13 is decreasing with increasing stellar luminosity among the lower luminosity giants in our sample.