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Astron. Astrophys. 329, 937-942 (1998)

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1. Introduction

Since globular clusters (GCs) are among the oldest objects in galaxies, they are widely recognized as very useful tracers of the chemical and dynamical evolution of their parent hosts.

The accurate knowledge of their global metal content, measured by the [Fe/H] ratio, is critical for many astrophysical problems. In particular, being very massive and luminous systems of coeval stars that show, to a first approximation, a similar (initial) chemical composition, globular clusters represent the cornerstones in establishing the existence of an age-metallicity relation and/or a metallicity-galactocentric distance gradient, up to the most distant regions of the galactic halo. This in turn provides strong constraints on models of galactic formation. Moreover, variations in the [Fe/H] content among globular clusters can be interpreted as a fossil record of the global processes of chemical enrichment occurred through the history of the Galaxy. Finally, precise metallicities are one of the basic ingredients in deriving accurate ages using parallaxes measured by the Hipparcos astrometry satellite (see Gratton et al. 1997; Reid 1997).

Even if the best way to get a quantitatively accurate estimate of the metal abundance of any star is detailed abundance analysis of high resolution spectra, there are unfortunately some shortcomings that limit the application of this technique to the study of GCs. Due to their large distances, reliable high resolution, high signal-to-noise spectra can be obtained with the present day instrumentation only for the brightest giants. Only an handful of stars near the main sequence turn-off (hence reflecting the initial chemical composition, undisturbed by mixing in later evolutionary phases) have been observed yet. Moreover, high-resolution spectroscopy is a very time consuming observing technique.

Therefore, in the past years, a number of indirect metallicity indicators have been devised to overcome these problems. Almost all of them are based on integrated parameters that bypass the distance limit also for very far clusters, but they require a very accurate calibration in order to provide the true content in [Fe/H]. A direct calibration, able to tie the observed photometric indices to the actual number of iron atoms as measured from high resolution spectral line profiles is henceforth strongly needed. In Sect. 2 we discuss the philosophy of our approach; Sects. 3 and 4 are devoted to the presentation and discussion of new calibrations for several metallicity indicators; a short summary is presented in Sect. 5.

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997