Snapshot metallicity estimate of resolved stellar systems through Lick Fe5270 diagnostic

We outline a new method to derive a "snapshot" metallicity estimate of stellar systems (providing one resolves at least the brightest part of the CMD) just on the basis of low-resolution (i.e., 6-8A FWHM) spectroscopy of a small stellar sample. Our method relies on the Fe5270 Lick index measurements and takes advantage of the special behavior of this spectral feature, that reaches its maximum strength among the ubiquitous component of K-type giants. This makes the Fe5270(max} estimate a robust and model-independent tracer of cluster [Fe/H], being particularly insensitive to the age of the stellar population. A comparison of the Fe5270(max) distribution derived from globular and open clusters, as well as from the field giant population in the Galaxy disk, confirms a tight correlation of the index maximum vs. cluster [Fe/H] allover the entire metallicity range for stellar population with [Fe/H] >~ -2.0. Relying on a theoretical calibration of the feature, we trust to effectively infer cluster metallicity within a typical uncertainty of 0.1-0.2dex, depending on RGB luminosity sampling of the observations.