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Astron. Astrophys. 350, 476-484 (1999)

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Strömgren photometry in globular clusters: M55 & M22 *

P. Richter 1, M. Hilker 2 and T. Richtler 1

1 Sternwarte der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
2 Departamento de Astronomía y Astrofísica, P. Universidad Católica, Casilla 104, Santiago 22, Chile

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 [FORMULA] dex for M55 and [FORMULA] dex for M22. The determination of metal abundances in cluster giants with the Strömgren [FORMULA] 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 [FORMULA], 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 [FORMULA] 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

* Based on data collected at the European Southern Observatory, La Silla, Chile

Send offprint requests to: prichter@astro.uni-bonn.de

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999