A calibration of Mg2 versus (Fe/H) and (Mg/Fe)

We have computed composite synthetic spectra in the wavelength range λλ490-540 nm representing globular clusters in the metallicity range - 2.0 < [Fe/H] < + 1.0. For the metal-rich ([Fe/H] > 0.0) clusters we adopted stellar populations similar to those of Galactic bulge globular clusters recently studied through color magnitude diagram analyses. We show that major points in defining the intensity of lines in the spectral region considered are the temperatures of red giants, and their [Mg/Fe] ratios. The Mg_2_ index was measured in the synthetic composite spectra, and plots of Mg_2_ versus [Fe/H] are given for the different assumptions. In particular, [Mg/Fe] is included as a parameter for the Mg_2_ calibration.