CN and CA abundance variations among the giants in M22

Abstract

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) is approximately equal to 0.08. Reanalysis of the data of Norris and Freeman (1983) indicates not only positive correlations between CH, CN, and Ca, but decidely bimodal distributions of CH and CN and a unimodal distribution for Ca. Our photometric indices, m1 and hk, demonstrate a range in metallicity that persists to two magnitudes below the horizontal branch, and confirms the correlation between calcium abundance and CN/CH. We infer from comparisons to spectroscopic data that m1 is dominated by the CN and CH abundance and find no independent evidence of a range in (Fe/H). The excessive ranges in m1 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.