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 F 110 W - F 160 W 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 onboard 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 composition .
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 \sim 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 .