The present article proposes new fitting formulas for the indices Mg₂ and Na D as functions of stellar parameters but including explicitly the abundance ratio of the relevant element. Integrated metallicity indices for single-stellar populations like, Mg₂, (Fe , Na D, Hβ, and (B - V) color have been computed for a grid of 100 models as a function of age, metallicity, and relative abundances which stimulate a medium enriched purely by type II supernova, or a mix of both types I and II. Different diagrams show model self-consistency, in that most globular clusters appear to have ages larger than 12 Gyr and an average abundance ratio [Mg/Fe]=0.34. Interpolation formulas based on our model computations are given for the different indices as functions of age, metallicity, and relative abundances.
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Borges et al. (1995) studied this question.