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Isotopic Carbon Abundances in M71
The Astronomical Journal
  • Michael M Briley, University of Wisonsin
  • Verne V Smith, University of Texas
  • Jeremy R King, Clemson University
  • David L Lambert, University of Texas
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The American Astronomical Society

High resolution echelle spectra of the 8005 Angstroms CN bands are presented for five bright giants in the globular cluster M71 (C1951 +186, NGC 6838). An analysis of the (13) CN feature included in this region confirms the results of \cite{briley94a}; that the M71 red giants evince low (12}C/({13)) C ratios and that these ratios are correlated with CN band strengths (CN-strong stars having lower (12}C/({13)) C ratios). This finding is consistent with the theory that the star-to-star CN band strength variations, as well as the oxygen and possibly sodium differences observed in M71, are related to the CN(O)-cycle of hydrogen burning. (One asymptotic giant branch candidate is also included in the present sample, which exhibits both weak-CN band strength and a low (12}C/({13)) C ratio.) Examination of published data on CN band strengths and (12}C/({13)) C ratios among the more metal-poor clusters implies a metallicity dependence in the efficiency of the underlying mechanism responsible for the low (12}C/({13)) C ratios. A likely candidate appears to be the mixing of CN(O)-processed material into the stellar atmospheres, although a primordial source of CN-processing cannot be ruled out.

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