Str\"omgren metallicities for intermediate-age and old star clusters

Abstract

We report results that show that the straightforwardly star cluster metallicities obtained from Str\"omgren $vby$ photometry is age-dependent and need to be corrected for further use. This outcome arises from the comparison of [Fe/H] values derived from Str\"omgren photometry with those metallicities published in the literature for 26 Large and Small Magellanic Cloud star clusters, whose ages range from $\sim$ 1 Gyr up to the known oldest globular clusters' ages in these galaxies. While deriving mean star cluster metallicities we carried out a thorough selection of red giant branch candidates to comply with the Str\"omgren metallicity calibration validity regime. We paid attention to the effect of contamination by field stars, particularly of those that lie inside the star clusters' radii, distributed along the star cluster red giant branches, and with [Fe/H] values covering a similar range as that for the selected stars. We found that the measured Str\"omgren metallicities are systematically more metal-poor than the published ones and that a quadratically age-varying function reproduces the relative metallicity values with an overall uncertainty of $\sim$ 0.05 dex. We finally performed a similar comparison relying on a fully independent approach, that consisted in using theoretical red giant branches of old globular clusters spanning [Fe/H] values from -2.0 up to 0.0 dex as standard ones. We then superimposed on to them the red giant branches of star clusters with ages in the range 1.0 - 12.5 Gyr and estimated by interpolation their associated metallicities. The derived theoretical relative metallicities follow a similar trend as a function of the star clusters' ages than that found from observations of star clusters.