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. , 6–8 Å 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 ] \gtrsim - 2.0 .
Relying on a theoretical calibration of the feature , we trust to effectively infer cluster metallicity within a typical uncertainty of 0.1–0.2 dex , depending on RGB luminosity sampling of the observations .

A handful of stars ( 5–10 objects ) is required for the method to be applied , with low-metallicity stellar populations more easily managed , being Fe5270 _ { max } located within the few brightest RGB stars of the system .
In any case , we show that even the observation of a coarse stellar set would allow us to place a confident lower limit to cluster metallicity .