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abstract
M22 is a low-latitude globular cluster with significant reddening ($E(B-V) \sim 0.42$, Crocker 1988) and evidence for a modest spread in extinction across the cluster field. It is also a cluster with some evidence for chemical inhomogeneity. We have applied extended Str\"omgren photometry ($uvbyCa$) to several fields in this cluster, focusing on a sample of $\sim 300$ giants, many of which have membership information from proper motions (Peterson \& Cudworth 1994) and quite a number of which have spectroscopic measurements for Fe, Ca, Na, CN and CH abundances. Our results strengthen and support the findings of Lehnert et al. (1991) and Norris \& Freeman (1983). Our photometric calcium index, which correlates well with spectroscopic indicators, shows a spread far larger than could be explained by differential reddening. The calcium metallicity index correlates well with the conventional Str\"omgren metal index $m_1$, and both are positively correlated with enhanced CN and CH as evidenced by S3839 and W(G) from Norris \& Freeman. Although a significant spread in $m_1$ exists, the spread in iron it implies is nearly an order of magnitude larger than observed spectroscopically, and it is suggested that variations in CN play the largest role in the dispersion of $m_1$ indices. M22 appears to be a cluster with an unusual history that, unlike $\omega$ Cen, has led to enhancements of $\alpha$--rich species with little variation in iron-peak elements.
