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Globular Clusters (GCs) in the Milky Way represent the ideal laboratory toestablish the age of the oldest stellar populations and to measure thecolor-magnitude relation of stars. Infrared (IR) photometry of these objectsprovides a new opportunity to accomplish this task. In particular, at lowstellar 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 thatlower mass and cooler dwarfs become bluer in the F110W - F160W color baselineand not redder. This inversion of the color-magnitude relation offers thepossibility to fit GC properties using IR imaging, and to reduce theiruncertainties. Here, we used the IR channel of the Wide Field Camera 3 onboardthe Hubble Space Telescope to obtain new, deep high-resolution photometry ofthe old metal-poor GC NGC6397. From the analysis of the GC CMD, we revealedbelow the MS "kink" the presence of two MSs with different chemicalcomposition. We derived the cluster fiducial line and we compared it with agrid of isochrones over a large range of parameter space, allowing age,metallicity, distance and reddening to vary freely within reasonable selectedranges. We derived an age of 12.6 Gyr with a random uncertainty sigma ~ 0.7Gyr. These results confirm that the analysis of the IR color-magnitude of starsprovide a valuable tool to measure the GC ages and offers a new venue todetermine their absolute age to sub-Gyr accuracy with next generation IRtelescopes.

The Age of the Old Metal-Poor Globular Cluster NGC6397 Using WFC3/IR Photometry