New age-metallicity diagnostic diagram for the Washington photometric system

PIATTI, ANDRÉS; PERREN, GABRIEL

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2015

0035-8711

The age calibration of the Washington $delta$T1 index is mainly used to estimate ages of star clusters older than 1 Gyr, no age-metallicity degeneracy effect is considered. We have profusely exploited synthetic T1 versus C − T1 colour magnitude diagrams aiming at exploring the intrinsic behaviour of the $delta$T1 index. The analysis shows that $delta$T1 varies with age and metal content as well. In general, the dependence on age weakens for ages greater than ∼ 6 Gyr, and results even less sensitive to age as the metallicity decreases. For ages younger than ∼ 5 Gyr $delta$T1 shows a strong correlation with both age and metallicity. The $delta$C index -defined as $delta$T1 for the C passband- is also a combined measurement of age and metallicity. We introduce a new age-metallicity diagnostic diagram, $delta$T1 versus $delta$C - $delta$T1, 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 the constraints found in the $delta$T1 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.