Mid-UV Narrow-Band Indices of Evolved Simple Stellar Populations
  We explore the properties of selected mid-ultraviolet (1900-3200 angstrom)
spectroscopic indices of simple stellar populations (SSPs). We incorporate the
high resolution UVBLUE stellar spectral library into an evolutionary population
synthesis code, based on the most recent Padova isochrones. We analyze the
trends of UV indices with respect to age and chemical composition. As a first
test against observations, we compare our results with the empirical mid-UV
spectral indices of Galactic globular clusters, observed with the International
Ultraviolet Explorer (IUE).
  We find that synthetic indices exhibit a variety of properties, the main one
being the slight age sensitivity of most of them for ages>2 Gyr. However, for
high metallicity, two indices, Fe II 2332 and Fe II 2402, display a remarkably
different pattern, with a sharp increase within the first two Gyr and,
thereafter, a rapid decline. These indices clearly mark the presence of young
(~1 Gyr) metal rich (Z > Z_sun) stellar populations.
  We complement existing UV indices of Galactic globular clusters with new
measurements, and carefully identify a sub-sample of ten indices suitable for
comparison with theoretical models. The comparison shows a fair agreement and,
in particular, the strong trend of the indices with metallicity is well
reproduced.
  We also discuss the main improvements that should be considered in future
modelling concerning, among others, the effects of alpha-enhancement in the
spectral energy distributions.
