Extracting parameters from colour–magnitude diagrams

Abstract

We present a simple approach for obtaining robust values of astrophysical parameters from the observed colour–magnitude diagrams (CMDs) of star clusters. The basic inputs are the Hess diagram built with the photometric measurements of a star cluster and a set of isochrones covering wide ranges of age and metallicity. In short, each isochrone is shifted in apparent distance modulus and colour excess until it crosses over the maximum possible Hess density. Repeating this step for all available isochrones leads to the construction of the solution map, in which the optimum values of age and metallicity – as well as foreground/background reddening and distance from the Sun – can be searched for. Controlled tests with simulated CMDs show that the approach is efficient in recovering the input values. We apply the approach to the open clusters M 67, NGC 6791 and NGC 2635, which are characterized by different ages, metallicities and distances from the Sun.