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.
