1995A&A...303..753S

An average integrated spectrum of the nearly-solar metallicity globular clusters NGC6528, NGC6553 and NGC6440 is synthesized in the range 3500<λ<9750A with a library of stellar spectra and constraints provided by the observed colour-magnitude diagram (CMD) of NGC6553. In order to account for the CMD parts below the observational cutoff, we employ a power law mass function with slopes x=-0.5, 1.35 and 2.5. We conclude that the MS contribution amounts to ≃15% of the integrated light at 5870A and this minor contribution makes the integrated spectrum basically insensitive to these x values. The dominant components are the Red Giant Branch and the Horizontal Branch with respectively ≃60% and ≃20% of the flux at λ5870A. The residuals of the observed integrated spectrum with respect to the model suggest that [Fe/H] is slightly subsolar in these globular clusters, whereas [CNO/Fe] is slightly above the solar ratio.