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The cyanogen distribution of M4 and the possible connection between horizontal branch morphology and chemical inhomogeneity
Norris, J.
AA(Mount Stromlo and Siding Spring Observatoires, Canberra, Australia)
Astrophysical Journal, Part 1, vol. 248, Aug. 15, 1981, p. 177-188. (ApJ Homepage)
Publication Date:
NASA/STI Keywords:
Abundance, Cyanogen, Globular Clusters, Horizontal Branch Stars, Stellar Evolution, Stellar Rotation, Angular Velocity, Inhomogeneity, Late Stars, Star Distribution, Stellar Mass Ejection, Stellar Spectra, Stellar Structure
Bibliographic Code:


A spectroscopic survey of 45 red giants in the globular cluster M4 has been completed with a view to ascertaining whether the bimodal distribution of stars on the horizontal branch (Lee) is accompanied by a cyanogen dichotomy on the giant branch, similar to the situation found in NGC 6752. From analysis of some 118 spectra, it is concluded that the red giants in M4 (in the magnitude range MV ˜0.3 to -1.2) do show a bimodal cyanogen distribution. There appears also to be an anticorrelation between the behavior of CN and CH. A working hypothesis is proposed which will explain most of the known peculiarities of the three globular clusters 47 Tuc, M4, and NGC 6752, for which comprehensive cyanogen surveys are available. it is suggested that there is a spectrum of core rotational velocities in the main-sequence stars of globular clusters. Beyond some critical value of the rotational velocity, a star will mix the products of the CN cycle into its outer layers, while still on or near the main sequence. The range in angular momentum leads also to a range in luminosity at which helium flash occurs and, thereby, to a range in envelope mass on the horizontal branch. Both the distribution of the products of mixing in the red giants and that of stars along the horizontal branch are thus determined by the spectrum of angular velocities in the cluster stars. This work thus supports the contention that core rotation is a parameter which effects horizontal branch morphology (the case is made that it should be regarded as the third parameter). The hypothesis has the advantage that it makes several predictions which are open to observational test.

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