Modeling the LICK/IDS Spectral Feature Indices Using Synthetic Spectra

Abstract

A group at Lick Observatory has measured spectral indices for stars, globular clusters, and galaxies. As part of a program to compute integrated spectra and colors for clusters and galaxies, we have modeled these indices for stars. This comparison is intended in part as a test of the synthetic indices and their value in modeling the other objects and in part as an attempt to obtain new insights, such as the dependence of the indices on the abundances of individual elements. Since we have shown previously that a 5 Gyr isochrone matches the color-magnitude diagram of M67 very well [AJ, 106, 618 (1993)], we have calculated synthetic spectral indices for temperature and gravity points along this isochrone. Our values, plotted as index versus temperature diagrams, have been compared with observations of M67 stars and field stars, the latter mostly being objects whose atmospheric parameters are similar to the isochrone values. These comparisons show generally good agreement and indicate that model cluster and galaxy spectra should yield valuable insights when compared to the corresponding observations. We studied the changes of the indices produced by changes in bandpasses and smoothing, results which may be of value to other observers, and to changes in overall metal abundance and in changes of the abundances of individual elements. While these latter changes give specific values for results which would be anticipated, such as the dependence of the Na D index on sodium abundance, they also show that many other indices vary only weakly with abundance or vary a great deal in an unexpected way. For example, the Fe4668 index is very sensitive to changes in carbon abundance. In addition, it is difficult to find the Fe abundance using the Lick indices, only one index being more sensitive to changes in Fe itself than to changes in the overall metal abundance.