7 Conclusions

For the first time the complete set of Lick indices have been measured for a sample of metal-rich globular clusters belonging to the Galactic bulge. In combination with data for metal-poor globular clusters this data set has allowed us to establish an empirical calibration of the Lick indices of old stellar populations from very low metallicities all the way to near solar metallicity. On the one hand, these empirical relations can be directly used to get age and chemical composition information for the stellar populations of unresolved galaxies. On the other hand, they can be used to submit to most stringent tests of population synthesis models, an aspect which is the subject of an accompanying paper (Maraston et al. 2002).

The comparison of the Lick indices for the Galactic bulge with those of globular clusters shows that the bulge and the most metal-rich globular clusters have quite similar stellar populations, with the slightly deviating values of some of the bulge indices being the likely result of the metallicity distribution of bulge stars, which extends down to $\rm [Fe/H]\simeq -1.0$ (McWilliam & Rich 1994; Zoccali et al. 2002). Within the uncertainties, both the metal-rich clusters and the bulge appear to have also the same index ratios, in particular those sensitive to [$\alpha$/Fe]. This implies similar enhancements for individual $\alpha$-elements in clusters as in the field. Existing spectroscopic determinations of the $\alpha$-element enhancement in clusters and bulge field stars are still scanty, but extensive high-resolution spectroscopy at 8-10 m class telescopes will soon provide data for a fully empirical calibration of the Lick indices at the [$\alpha$/Fe] values of the bulge and bulge globular clusters.

Some other line index ratios, such as CN$/\langle$Fe$\rangle$, show clear exceptions. In these cases the bulge indices are definitely below the values for the metal-rich clusters. Several possibilities have been discussed for the mechanism responsible for the CN index offset between the bulge and the clusters, the environmental-pollution being active in clusters (but not in the field) appearing as the most likely explanation. In this scenario, globular cluster stars would have experienced accretion of materials lost by cluster AGB stars, early in the history of the clusters (i.e., when clusters were ${\sim} 10^8{-}10^9$ years old).