Towards a photometric metallicity scale for open clusters
  Open clusters are a useful tool when investigating several topics connected
with stellar evolution; for example the age or distance can be more accurately
determined than for field stars. However, one important parameter, the
metallicity, is only known for a marginal percentage of open clusters. We aim
at a consistent set of parameters for the open clusters investigated in our
photometric Delta-a survey of chemically peculiar stars. Special attention is
paid to expanding our knowledge of cluster metallicities and verifying their
scale. Making use of a previously developed method based on normalised
evolutionary grids and photometric data, the distance, age, reddening, and
metallicity of open clusters were derived. To transform photometric
measurements into effective temperatures to use as input for our method, a set
of temperature calibrations for the most commonly used colour indices and
photometric systems was compiled. We analysed 58 open clusters in total. Our
derived metallicity values were in excellent agreement with about 30
spectroscopically studied targets. The mean value of the absolute deviations
was found to be 0.03 dex, with no noticeable offset or gradient. The method was
also applied using recent evolutionary models based on the currently accepted
lower solar abundance value Z=0.014. No significant differences were found
compared to grids using the former adopted solar value Z=0.02. Furthermore,
some divergent photometric datasets were identified and discussed. The method
provides an accurate way of obtaining properly scaled metallicity values for
open clusters. In light of present and future homogeneous photometric sky
surveys, the sample of stellar clusters can be extended to the outskirts of the
Milky Way, where spectroscopic studies are almost impossible. This will help
for determining galactic metallicity gradients in more detail.
