Document Type

Article

Publication Date

1-1-1997

Publication Title

The Astronomical Journal

Publisher

The American Astronomical Society

Abstract

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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