Synthesis of the integrated spectrum of metal-rich globular clusters using the hr diagram and a stellar library

Abstract

An average integrated spectrum of the nearlysolar metallicity globular clusters NGC 6528, NGC 6553 and NGC 6440 is synthesized in the range 3500 < λ < 9750Å with a library of stellar spectra and constraints provided by the observed colour-magnitude diagram (CMD) of NGC 6553. 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 5870Å 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 λ5870Å. The residuais 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.