A Technique for Simultaneous Determination of the Metallicity and Reddening of Old Open Clusters Using (V,B-V) Color-Magnitude Diagrams

A technique similar to that originally developed by Sarajedini (1994) for the simultaneous determination of [Fe/H] and E(B-V) in globular clusters, is applied to seven old open clusters in our Galaxy (NGC6791, Be32, Mel66, M67, NGC2420, To2 and IC4651). With these clusters we prove that there exist linear relationships between [Fe/H] and Delta V_{1.2} and [Fe/H] and (B-V)_{0,g}. From these relations we derive a general technique for the determination of metallicity and colour excess. The success obtained with its application supports it as a general-use technique. This procedure is also applied to the clusters Be39, King11 and NGC1193. The results obtained for the metallicity and colour excess values, are in excellent agreement with those published in the literature. Clusters Be29 and Be22, for which very uncertain values for metallicity and colour excess are published, are used with our technique. We determine values for [Fe/H]=-0.30, \ -0.42 and E(B-V)=0.01, \ 0.70 respectively. It seems that there is a transition region between globular and old open clusters around [Fe/H] ~ -0.4.