Strömgren photometry in globular clusters: M55 & M22 *
P. Richter 1,
M. Hilker 2 and
T. Richtler 1
Received 6 April 1999 / Accepted 13 July 1999
We present Strömgren CCD photometry for the two galactic globular clusters M55 (NGC 6809) and M22 (NGC 6656).
We find average Strömgren metallicities of dex for M55 and dex for M22. The determination of metal abundances in cluster giants with the Strömgren 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 giant-branch 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 , 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 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.
Key words: Galaxy: globular clusters: individual: M22 Galaxy: globular clusters: individual: M55 stars: abundances stars: blue stragglers
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© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999