2015MNRAS.450.3771P

The age calibration of the Washington δT1 index is mainly used to estimate ages of star clusters older than 1 Gyr and no age-metallicity degeneracy effect is considered. We have profusely exploited synthetic T1 versus C - T1 colour magnitude diagrams to explore the intrinsic behaviour of the δT1 index. The analysis shows that δT1 varies with age and metal content as well. In general, the dependence on age weakens for ages greater than ∼ 6 Gyr, and is even less sensitive to age as the metallicity decreases. For ages younger than ∼ 5 Gyr, δT1 shows a strong correlation with both age and metallicity. The δC index - defined as δT1 for the C passband - is also a combined measurement of age and metallicity. We introduce a new age-metallicity diagnostic diagram, δT1 versus δC - δT1, which can unambiguously provide age and metallicity estimates, simultaneously. The new procedure can be used to derive ages from 1 up to 13 Gyr and metallicities [Fe/H] from -2.0 up to +0.5 dex, and is independent of the cluster reddening and distance modulus. It solves the constraints found in the δT1 index and surpasses the performance of the standard giant branch metallicity method. All these features make the diagnostic diagram a powerful tool for estimating accurate ages as well as metallicities.