Implications of a mixing origin for globular cluster bimodal CN variations

Abstract

New spectroscopic data are presented that reveal that the red giants in the globular cluster NGC 6934 exhibit a bimodal CN distribution, as is common among intermediate-metallicity clusters. Clear evidence for an anticorrelation between the strengths of the lambda3883 CN and the lambda4300 CH bands is found. This observation suggests that the CN-rich NGC 6934 giants have mixed CN(O.)-processed material up into their atmospheres. An analytical formulation is then developed to investigate the behavior of the CN opacity as increasing amounts of CN-processed material are brought into the atmosphere. It is found that in the absence of significant CO formation the CN opacity reaches a maximum at a carbon depletion of approx.0.22 dex, a result that is confirmed by spectrum-synthesis calculations. The advent of significant CO formation in relatively cool (T/sub eff/< or approx. =4000 K) atmospheres causes the CN opacity to peak at smaller carbon depletions. These results are shown to imply that the CN-poor stars, in clusters that exhibit bimodal CN distributions, must be relatively unmixed. It is also found that a bimodal CN distribution within a globular cluster need not imply an underlying bimodal carbon-abundance distribution. In fact a monomodal carbon distribution, with a peak at themore » low carbon-depletion end, could produce a CN bimodality. The addition of reasonable amounts of both CN and ON-processed material into the atmosphere does not greatly alter the surface carbon depletion at which the CN opacity maximum occurs.« less