A statistical method to determine open cluster metallicities

Context. The study of open cluster metallicities helps to understand the local stellar formation and evolution throughout the Milky Way. Its metallicity gradient is an important tracer for the Galactic formation in a global sense. Because open clusters can be treated in a statistical way, the error of the cluster mean is minimized. Aims. Our final goal is a semi-automatic statistical robust method to estimate the metallicity of a statistically significant number of open clusters based on Johnson BV data of their members, an algorithm that can easily be extended to other photometric systems for a systematic investigation. Methods. This method incorporates evolutionary grids for different metallicities and a calibration of the effective temperature and luminosity. With cluster parameters (age, reddening and distance) it is possible to estimate the metallicity from a statistical point of view. The iterative process includes an intrinsic consistency check of the starting input parameters and allows us to modify them. We extensively tested the method with published data for the Hyades. Results. We selected sixteen open clusters within 1000 pc around the Sun with available and reliable Johnson BV measurements. In addition, Berkeley 29, with a distance of about 15 kpc was chosen. For several targets we are able to compare our result with published ones which yielded a very good coincidence (including Berkeley 29). Conclusions. A new method for the statistical determination of open cluster metallicities is presented and tested. It is quite robust against errors in effective temperature and luminosity calibration of the individual stars.