NEW SYNTHETIC MG-2 INDEXES

Using the most recent grid of Kurucz model atmospheres and his improve d set of numerical codes to compute synthetic stellar spectra, we have calculated the strength of the magnesium absorption feature at 5175 A ngstrom (Mg-2 index) from a set of 141 computed synthetic spectra. The index values refer to the effective temperature range 4000-6000 K, su rface gravity from 1.5 to 4.5 dex, and metallicity from -0.5 to 1.0 de x. The computed index for the solar model matches very well the value obtained from observational data. Above 5000 K, the new Mg-2 values an d previous calculations agree to within 0.02 mag, but discrepancies ca n reach almost 0.58 mag for cool, metal-rich, low-gravity stars, so th at large values of Mg-2 previously obtained for low temperature dwarfs are now obtained for giants as well. A comparison with observational data is also presented.