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Session 82 - Globular Clusters.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
High resolution echelle spectra of the 8005 ÅCN feature in five M71 bright red giants are presented. A synthetic spectrum analysis of the ^13CN feature in this region yields ^12C/^13C 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 ^12C/^13C ratios are also found to be correlated with CN band strengths (CN-strong stars having lower ^12C/^13C ratios) and anticorrelated with O abundances (O-poor stars having lower ^12C/^13C 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 ^12C/^13C ratios observed in more metal-poor cluster giants reveals a trend of decreasing ^12C/^13C 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 ^12C/^13C ratio.
This work was partially supported by a grant from the Fund for Astrophysical Research.
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