NEW SYNTHETIC MG-2 INDEXES

Using the most recent grid of Kurucz model atmospheres and his improved set of numerical codes to compute synthetic stellar spectra, we have calculated the strength of the magnesium absorption feature at 5175 Angstrom (Mg-2 index) from a set of 141 computed synthetic spectra. The index values refer to the effective temperature range 4000-6000 K, surface gravity from 1.5 to 4.5 dex, and metallicity from -0.5 to 1.0 dex. The computed index for the solar model matches very well the value obtained from observational data. Above 5000 K, the new Mg-2 values and previous calculations agree to within 0.02 mag, but discrepancies can reach almost 0.58 mag for cool, metal-rich, low-gravity stars, so that 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.