Model-independent diagnostics of highly reddened Milky Way star clusters: age calibration

Context: The next generation near- and mid-infrared Galactic surveys will yield a large number of new highly obscured star clusters. Detailed characterization of these new objects with spectroscopy is time-consuming.
Aims: Diagnostic tools that will be able to characterize clusters based only on the available photometry will be needed to study large samples of the newly found objects.
Methods: The brightness difference between the red clump and the main-sequence turn-off point have been used as a model-independent age calibrator for clusters with ages from a few 108 to 1010 yr in the optical. Here we apply for the first time the method in the near-infrared.
Results: We calibrated this difference in K-band, which is likely to be available for obscured clusters, and we apply it to a number of test clusters with photometry comparable to the one that will be yielded by the current or near-future surveys.
Conclusions: The new calibration yields reliable ages over the range of ages for which the red clump is present in clusters. The slope of the relation is smoother than that of the corresponding V-band relation, reducing the uncertainty in the age determinations with respect to the optical ones.