BVI photometry of M22

The study is complemented by the photometry in two narrowband filters centered on H{alpha} and the adjacent continuum, and by infrared J, H and K magnitudes derived from the 2 MASS survey for ~2000 stars. Profiting from this huge database, we completely characterized the evolved stellar sequences of the cluster by determining a variety of photometric parameters, including new photometric estimates of the mean metallicity, reddening and distance to the cluster.

In particular, from our multi-wavelength analysis, we re-examined the long-standing metallicity spread problem in M 22. According to our dataset, we conclude that most of the observed width of the red giant branch must be due to differential reddening, which amounts to a maximum of {Delta}E(B-V)~0.06, although the presence of a small metallicity spread cannot be completely ruled out. More specifically, the maximum metallicity spread allowed by our data is of the order of {Delta}[Fe/H]~0.1-0.2dex, i.e., not much more than what allowed by the photometric errors.