A new method to estimate the ages of globular clusters: The case of NGC 3201

Abstract

We devised a new method to estimate globular cluster absolute ages by adopting the knee of the bending of the lower main-sequence (MS) in the Near-Infrared (NIR) J, J-Ks color-magnitude diagram. The color difference between this feature and the Turn-Off point is strongly correlated to the cluster age. This method is marginally affected by distance and reddening uncertainties, and by the possible occurrence of differential reddening. Furthermore, the knee location does not depend on the cluster age and it is a robust theoretical prediction. We adopted accurate J, Ks-band photometry collected with both MAD/VLT and SOFI/NTT for the Galactic globular cluster NGC 3201 to identify the location of the knee at J~19.90 ±0.03 and J-Ks ~0.76±0.02 mag. The comparison with different sets of cluster isochrones, transformed adopting different Color-Temperature-Relations (CTRs), shows that the models are slightly redder than the observations for J > 19 mag. This difference could be due to the presence of a calibration drift or to a problem of the CTRs in this magnitude range.