Basing on the huge library of 1-A resolution spectra calculated by Munari et
al. over a large range of logT, log g, [Fe/H] and both for solar and a-enhanced
abundance ratios [a/Fe], we present theoretical absorption-line indices on the
Lick system. First we derive the so-called response functions (RFs) of Tripicco
& Bell for a wide range of logT, log g, [Fe/H] and [a/Fe]=+0.4 dex. The RFs are
commonly used to correct indices with solar [a/Fe] ratios to indices with
[a/Fe]>0. Not only the RFs vary with the type of star but also with the
metallicity. Secondly, with the aid of this and the fitting functions (FFs) of
Worthey et al., we derive the indices for single stellar populations and
compare them with those obtained by previous authors, e.g. Tantalo & Chiosi.
The new RFs not only supersede the old ones by Tripicco & Bell, but also show
that Hb increases with the degree of enhancement in agreement with the results
by Tantalo & Chiosi. The new indices for single stellar populations are used to
derive with aid of the recursive Minimum Distance method the age, metallicity
and degree of enhancement of a sample of Galactic Globular Clusters for which
these key parameters have been independently derived from the Colour-Magnitude
Diagram and/or spectroscopic studies. The agreement is remarkably good.Comment: 18 pages, to appear in Astronomy & Astrophysic
