New age-metallicity diagnostic diagram for the Washington photometric system

A.E. PIATTI; G.I. PERREN

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2015 vol. 450 p. 3771 - 3777

0035-8711

The age calibration of the Washington $delta$$T_1$ index is mainly used to estimate agesof star clusters older than 1 Gyr, no age-metallicity degeneracy effect is considered.We have profusely exploited synthetic $T_1$ versus $C-T_1$ colour magnitude diagrams aiming at exploring the intrinsic behaviour of the $delta$$T_1$ index. The analysis shows that $delta$$T_1$ varies with age and metal content as well. In general,the dependence on age weakens for ages greater than $sim$ 6 Gyr, andresults even less sensitive to age as the metallicity decreases.For ages younger than $sim$ 5 Gyr $delta$$T_1$ shows a strong correlation withboth age and metallicity. The $delta$$C$ index -defined as $delta$$T_1$for the $C$ passband- is also a combined measurement of age and metallicity.We  introduce a new age-metallicity diagnostic diagram, $delta$$T_1$ versus$delta$$C$ - $delta$$T_1$, which has shown the ability of unambiguously providing age and metallicity estimates,  simultaneously. The new procedure allows to derive ages from 1 up to 13 Gyr and metallicities [Fe/H] from -2.0 up to +0.5 dex, and is independent of the cluster reddening and distance modulus. It does solve theconstraints found in the $delta$$T_1$ index and surpasses the performance of thestandard giant branch metallicity method. All these features make the diagnostic diagram  a powerful tool for estimating accurate ages as well as metallicities.