1995PASP..107...32A

We have obtained uvbyCa data for over 300 giants and horizontal branch stars in three fields of M22. The spread in (b-y) for the giant and horizontal branches is consistent with a spread in foreground reddening Delta-E (B-V) ∼0.08. Reanalysis of the data of Norris and Freeman (1983) indicates not only positive correlations between CH, CN and a unimodal distribution for Ca. Our photometric indices, m_1, and hk, demonstrate a range in metallicity that persists to two magnitudes below the horizontal branch, and confirm the correlation between calcium abundance and CN/CH. We infer from comparisons to spectroscopic data that m_1 is dominated by the CN and CH abundance and find no independent evidence of a range in [Fe/H]. The excessive ranges in m_1 and hk also suggest the influence of a continuous opacity source, reminiscent of the Bond-Neff effect, that is correlated with CNO abundance. The relative contributions of internal mixing and primordial variations for M22's giants are discussed.