Carbon Isotopes in M71 Giants - CNO, Mixing, and Metallicity

Details Carbon Isotopes in M71 Giants - CNO, Mixing, and Metallicity Carbon Isotopes in M71 Giants - CNO, Mixing, and Metallicity

Dec 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aas...187.8208b&link_type=abstract

American Astronomical Society, 187th AAS Meeting, #82.08; Bulletin of the American Astronomical Society, Vol. 27, p.1405

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Scientific paper

High resolution echelle spectra of the 8005 Angstroms CN feature in five M71 bright red giants are presented. A synthetic spectrum analysis of the (13) CN feature in this region yields (12}C/({13)) C ratios considerably lower than those expected from standard first dredge-up models for all five stars. The low ratios imply substantial exposure of envelope material to the CN and/or CNO-cycle of H-burning. This is a well known contradiction between theory and observation in the case of evolved low mass stars and thought to be the result of some "extra" mixing process operating after the first dredge-up phase of red giant evolution. The (12}C/({13)) C ratios are also found to be correlated with CN band strengths (CN-strong stars having lower (12}C/({13)) C ratios) and anticorrelated with O abundances (O-poor stars having lower (12}C/({13)) C ratios). This is entirely consistent with the suggestion that the star-to-star CN band strength variations, and to a lesser extent O abundance differences, observed in M71 and other globular cluster bright red giants are associated with H-burning via the CN(O)-cycle. A comparison with (12}C/({13)) C ratios observed in more metal-poor cluster giants reveals a trend of decreasing (12}C/({13)) C ratios with decreasing metallicity. This suggests a metallicity dependence to the efficiency of any underlying mixing mechanism and a natural explanation for the smaller star-to-star variations in [O/Fe] and [Na/Fe] seen in M71 by Sneden et al. (1994, AJ, 107, 1773) when compared to its more metal-poor counterparts (M3, M13, M10, etc.). One asymptotic giant branch candidate is included in the present sample, which exhibits both weak-CN band strength and a low (12}C/({13)) C ratio. This work was partially supported by a grant from the Fund for Astrophysical Research.

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