Strömgren photometry in globular clusters : M 55 & M 22 ⋆

We present Strömgren CCD photometry for the two galactic globular clusters M55 (NGC 6809) and M22 (NGC 6656). We find average Strömgren metallicities of −1.71±0.04 dex for M55 and −1.62 ± 0.08 dex for M22. The determination of metal abundances in cluster giants with the Strömgren m1 index in comparison with spectroscopic data from Briley et al. (1993) and Norris & Freeman (1982, 1983) shows that M55 and M22 have different distributions of cyanogen strengths. In M55, no CN abundance variations are visible among the giantbranch stars. In striking contrast, a large dispersion of cyanogen strengths is seen in M22. For M22 we find patchily distributed variations in the foreground reddening of ∆E(B − V ) ≈ 0.07, which explain the colour dispersion among the giant-branch stars. There is no evidence for a spread in iron within M22 since the variations in m1 are dominated by the large range in CN abundances, as already found by Anthony-Twarog et al. (1995). The difference between M55 and M22 may resemble the difference in integral CN band strength between M31 globular clusters and the galactic system. The colour-magnitude diagram of M55 shows the presence of a population of 56 blue-straggler stars that are more centrally concentrated than the red giant-branch stars.