An empirical calibration of Lick indices using Milky Way Globular
  Clusters
  To provide an empirical calibration relation in order to convert Lick indices
into abundances for the integrated light of old, simple stellar populations for
a large range in the observed [Fe/H] and [alpha/Fe]. This calibration
supersedes the previously adopted ones because it is be based on the real
abundance pattern of the stars instead of the commonly adopted metallicity
scale derived from the colours. We carried out a long-slit spectroscopic study
of 23 Galactic globular cluster for which detailed chemical abundances in stars
have been recently measured. The line-strength indices, as coded by the Lick
system and by Serven et al. (2005), were measured in low-resolution integrated
spectra of the GC light. The results were compared to average abundances and
abundance ratios in stars taken from the compilation by Pritzl et al. (2005) as
well as to synthetic models. Fe-related indices grow linearly as a function of
[Fe/H] for [Fe/H]>-2. Mg-related indices respond in a similar way to [Mg/H]
variations, however Mgb turns out to be a less reliable metallicity indicator
for [Z/H]<-1.5 . Despite the known Mg overabundance with respect to Fe in GC
stars, it proved impossible to infer a mean [Mg/Fe] for integrated spectra that
correlates with the resolved stars properties, because the sensitivity of the
indices to [Mg/Fe] is smaller at lower metallicities. We present empirical
calibrations for Ca, TiO, Ba and Eu indices as well as the measurements of
H_alpha and NaD.
