Astron. Astrophys. 350, 476-484 (1999)

5. Discussion and conclusions

A comparative study of the two galactic globular clusters M55 and M22 has shown that the Strömgren system can efficiently detect differences in the CN-strength distribution of globular clusters. Moreover, with the method presented in Sect. 3.2 Strömgren photometry can be used for the simultaneous determination of mean metallicities and reddening values in globular clusters.

The present study of M55 and M22 has revealed that the clusters are very different regarding the distribution of Strömgren colours. M55 turns out to be very uniform with respect to Strömgren-metallicities. The sharp red giant branch in the CMD as well as in the diagram indicates the chemical homogeneity of this globular cluster.

For M22, we confirm the results by Anthony-Twarog et al. (1995). A large scatter in the Strömgren , diagram indicates a large spread in CN strengths among red giants. Therefore, Strömgren colours of M22 stars cannot be interpreted in terms of an overall metallicity, in particular nothing can be said concerning an intrinsic metallicity variation among M22 stars. Moreover, the colour spread of the RGB and the BHB in the colour-magnitude diagram of M22 is not caused by a metallicity variation but probably by patchy reddening variations in the order of over the cluster area, because most of the reddest giant-branch stars are spatially concentrated in the southern part of the cluster. This, and the fact that there is no correlation between the dispersions in the CMD and the diagram, led us to conclude that a spread in [Fe/H], if present, is of minor importance with respect to the CN scatter. Considering similar results for M22 of Anthony-Twarog et al. (1995) and the spectroscopic measurements by Lehnert et al. (1991), Cen remains as the only globular cluster in the Milky Way, where significant evidence exists for a possibly primordial dispersion in [Fe/H].

So far, the existence of a wide range of cyanogen strengths has been shown only in a few globular clusters (see Kraft 1994 for a detailed overview) and their origin can be explained by primordial as well as by evolutionary origin. See to this also the extensive discussion in Anthony-Twarog et al. (1995). Perhaps, CN-rich clusters may be brought into relation with anomalies found in the comparison of integrated spectra of M31 clusters with those of galactic globular clusters. Several authors found (Burstein et al. 1988; Brodie & Huchra 1990) that, at the same metallicity, M31 clusters showed stronger CN-features than galactic clusters. Unfortunately, no work exists measuring integral CN strengths for both M22 and M55.

It is of obvious interest to perform a larger census of the CN-strength distribution among globular-cluster stars. The Strömgren system proved to be an efficient tool for this purpose. At present, it is not possible to investigate the relation of "CN-richness" with any other properties of globular clusters. M55 and M22 are perhaps representatives of two classes of globular clusters regarding their CN-strength distribution. If the CN-strengths in M22 are decoupled from stellar evolution, what else can be the cause? A striking difference between M55 and M22 is their stellar density. M55 is one of the most loosely structured globular clusters while, in contrast, M22 has a quite high density. It is imaginable that encounters or merging of stars can change the CN-surface abundance of red giants. It is therefore of high interest to study a larger sample of globular clusters.

© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999
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