On the metallicity of open clusters^*

I. Photometry

¹ Institut für Astronomie der Universität Wien, Türkenschanzstr. 17, 1180 Wien, Austria e-mail: Ernst.Paunzen@univie.ac.at
² Department of Physics and Astronomy, Uppsala University, Box 515, 75120 Uppsala, Sweden
³ Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kačićeva 26, 10000 Zagreb, Croatia
⁴ Université Bordeaux 1, CNRS, Laboratoire d'Astrophysique de Bordeaux, BP 89, 33270 Floirac, France

Received: 25 January 2010
Accepted: 22 March 2010

Abstract

Context. Metallicity is one of four free parameters typically considered when fitting isochrones to the cluster sequence. Unfortunately, this parameter is often ignored or assumed to be solar in most papers. Hence an unknown bias is introduced in the estimation of the other three cluster parameters (age, reddening and distance). Furthermore, studying the metallicity of open clusters allows us not only to derive the Galactic abundance gradient on a global scale, but also to trace the local solar environment in more detail.

Aims. In a series of three papers, we investigate the current status of published metallicities for open clusters from widely different photometric and spectroscopic methods. A detailed comparison of the results allows us to establish more reliable photometric calibrations and corrections for isochrone fitting techniques. Well established databases such as WEBDA help us to perform a homogeneous analysis of available measurements for a significant number of open clusters.

Methods. The literature was searched for [Fe/H] estimates on the basis of photometric calibrations in any available filter system. On the basis of results published by Tadross, we demonstrate the caveats of the calibration choice and its possible impact. In total, we find 406 individual metallicity values for 188 open clusters within 64 publications. The values were, finally, unweightedly averaged.

Results. Our final sample includes [Fe/H] values for 188 open clusters. Tracing the solar environment within 4000 × 4000 pc² we identify a patchy metallicity distribution as an extension to the Local Bubble that significantly influences the estimation of the Galactic metallicity gradient, even on a global scale. In addition, further investigations of more distant open clusters are clearly needed to obtain a more profound picture at Galactocentric distances beyond 10 000 pc.

Conclusions. Only a combination of all available photometric and spectroscopic data will shed more light on how the local and global Galactic properties are correlated with metallicity.