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There now exists an enormous, homogeneous database of absorption-feature indices measured from Lick Observatory IDS spectra of field stars, cluster stars, galaxies and globular clusters. These data will be invaluable in interpreting integrated-light spectra, thus furthering our understanding of stellar populations in clusters and E galaxies. The interpretation of these data, however, is limited by our knowledge regarding the variation of the indices as a function of temperature, gravity, and chemical composition. This has been examined empirically, most recently by Worthy et al. 1994 (ApJS,94,687), but addressing such key aspects as the effect of non-solar abundance ratios is not feasible observationally.
In order to interpret these data in more detail we measure the Lick/IDS indices from a T$_{\rm eff}$/log g/[Fe/H] grid of synthetic stellar spectra, as well as from spectra calculated at intervals along theoretical isochrones. Our fit to the various Lick/IDS indices has been optimized by comparing high-resolution spectra of the Sun and of Arcturus with our models for these objects.
We investigate the behavior of the Lick/IDS indices in response to variations of the input physical parameters. Of particular interest are those involving changes in the relative abundances of the CNO elements as well as magnesium and other alpha-process elements. For example, we find that the Fe~4668 index is extremely sensitive to [C/Fe] due to absorption from the Swan bands of molecular carbon. We also calibrate the widely used Mg$_2$ index as a function of [Mg/Fe].
