Carbon Isotope Ratios on the Upper Red Giant Branch of Messier 7111The data presented here were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.

Abstract

ABSTRACTThis 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 MV<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 MV<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<MV<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/13C is decreasing with increasing stellar luminosity among the lower luminosity giants in our sample.