On the metallicity of open clusters - I. Photometry

摘要

Context. Metallicity is one of four free parameterstypically considered when fitting isochrones to the cluster sequence.Unfortunately, this parameter is often ignored or assumed to be solarin most papers. Hence an unknown bias is introduced in the estimationof the other three cluster parameters (age, reddening and distance).Furthermore, studying the metallicity of open clusters allows us notonly to derive the Galactic abundance gradient on a global scale, butalso to trace the local solar environment in more detail. Aims. In a series of three papers, we investigate the currentstatus of published metallicities for open clusters from widelydifferent photometric and spectroscopic methods. A detailed comparisonof the results allows us to establish more reliable photometriccalibrations and corrections for isochrone fitting techniques. Wellestablished databases such as WEBDA help us to perform a homogeneousanalysis of available measurements for a significant number of openclusters. Methods. The literature was searched for [Fe/H] estimates on thebasis of photometric calibrations in any available filter system. Onthe basis of results published by Tadross, we demonstrate the caveatsof the calibration choice and its possible impact. In total, we find406 individual metallicity values for 188 open clusters within 64publications. The values were, finally, unweightedly averaged. Results. Our final sample includes [Fe/H] values for 188 open clusters. Tracing the solar environment within pc2we identify a patchy metallicity distribution as an extension to theLocal Bubble that significantly influences the estimation of theGalactic metallicity gradient, even on a global scale. In addition,further investigations of more distant open clusters are clearly neededto obtain a more profound picture at Galactocentric distances beyond10000pc. Conclusions. Only a combination of all available photometric andspectroscopic data will shed more light on how the local and globalGalactic properties are correlated with metallicity. Key words: Galaxy: abundances - open clusters and associations: general - stars: abundances