Towards a photometric metallicity scale for open clusters

Abstract

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.

Accepted for publication in A&A (19 pages including online material)

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• New atmospheric parameters and spectral interpolator for the MILES cool stars
• A comparative study on the reliability of open cluster parameters
• The Binary Fraction and Mass Segregation in Alpha Persei Open Cluster
• Metallicity calibrations for dwarf stars and giants in the Geneva photometric system
• Determining the seismic age of the young open cluster $\alpha$ Per using $\delta$ Scuti stars
• Chemical composition of intermediate mass stars members of the M6 (NGC 6405) open cluster
• The origin of early-type runaway stars from open clusters
• Characterizing dynamical stages of open clusters located in the Sagittarius spiral arm
• Study of the Open Clusters in Kepler Prime Field
• A search for peculiar stars in the open cluster Hogg 16