The Age of the Old Metal-poor Globular Cluster NGC 6397 Using WFC3/IR Photometry

Globular clusters (GCs) in the Milky Way represent the ideal laboratory to establish the age of the oldest stellar populations and to measure the color magnitude relation of stars. Infrared (IR) photometry of these objects provides a new opportunity to accomplish this task. In particular, at low stellar masses, the stellar main sequence (MS) in an IR color magnitude diagram (CMD) exhibits a sharp "kink" (due to opacity effects in M dwarfs), such that lower mass and cooler dwarfs become bluer in the F110W - F160W color baseline and not redder. This inversion of the color magnitude relation offers the possibility to fit GC properties using IR imaging, and to reduce their uncertainties. Here, we used the IR channel of the Wide Field Camera 3 on board the Hubble Space Telescope to obtain new, deep high-resolution photometry of the old metal-poor GC NGC 6397. From the analysis of the GC CMD, we revealed below the MS "kink" the presence of two MSs with different chemical compositions. We derived the cluster fiducial line and we compared it with a grid of isochrones over a large range of parameter space, allowing age, metallicity, distance, and reddening to vary freely within reasonable selected ranges. We derived an age of 12.6 Gyr with a random uncertainty sigma similar to 0.7 Gyr. These results confirm that the analysis of the IR color magnitude of stars provide a valuable tool to measure the GC ages and offers a new venue to determine their absolute age to sub-Gyr accuracy with next generation IR telescopes.