1995A&A...303...79C

Spectrophotometric indices for 18 Galactic globular clusters, obtained from CCD observations and careful reductions, were used to determine reliable calibrations on metallicity [Fe/H]. The indices were measured in the bandpasses adopted by Burnstein et al. (1984). Adding other observations of Burnstein et al. (1984) we obtained our results from an homogeneous sample of indices for 26 globular clusters. Relations with indices defined by other Authors and with metallicity photometric indices or parameters were also computed. In each case the relations are quite satisfactory. Observational data were compared with synthetic indices derived from Buzzoni's (1989) models and detailed discussions were performed for Mg₂, Fe₅₂, and H_β. The observational points seem to be systematically shifted with respect to the fiducial lines traced by the models. The scenario confirms that a certain degree of oxygen enhancement would be necessary to obtain a better agreement between observed data and theoretical predictions. This enhancement, however, removes some of the disagreement, but not all of it. The dependence of the observed Fe₅₂ and H_β indices on the metal content for different HB morphologies was considered. Finally, some results were also discussed from a statistical point of view. A principal component analysis was applied to the index sample to study the number of independent parameters necessary to reproduce the observations. The whole index set is completely consistent with a one-parameter family.