COMPREHENSIVE STELLAR POPULATION-MODELS AND THE DISENTANGLEMENT OF AGE AND METALLICITY EFFECTS

The construction of detailed models for intermediate and old stellar populations is described. Input parameters include metallicity (-2 < [Fe/H] (0.5), single-burst age (between 1.5 and 17 Gyr), and initial mass function (IMF) exponent. Quantities output include broadband magnitudes, spectral energy distributions, surface brightness fluctuation magnitudes, and a suite of 21 absorption feature indices. The models are checked against a wide variety of available observations. Examinations of model output yield the following conclusions. (1) If the percentage change Delta age/Delta Z approximate to 3/2 for two populations, they will appear almost identical in most indices. A few indices break this degeneracy by being either more abundance sensitive (Fe4668, Fe5015, Fe5709, and Fe5782) or more age sensitive (G4300, H beta, and presumably higher order Balmer lines) than usual. (2) Present uncertainties in stellar evolution are of the same magnitude as the effects of IMF and Y in the indices studied. (3) Changes in abundance ratios (like [Mg/Fe]) are predicted to be readily apparent in the spectra of old stellar populations. (4) The I-band flux of a stellar population is predicted to be nearly independent of metallicity and only modestly sensitive to age. The I band is therefore recommended for standard candle work or studies of M/L in galaxies. Other conclusions stem from this work. (1) Intercomparison of models and observations of two TiO indices seem to indicate variation of the [V/Ti] ratio among galaxies, but it is not clear how this observation ties into the standard picture of chemical enrichment. (2) Current estimates of [Fe/H] for the most metal-rich globulars that are based on integrated indices are probably slightly too high. (3) Colors of population models from different authors exhibit a substantial range. At solar metallicity and 13 Gyr, this range corresponds to an age error of roughly +/-7 Gyr. Model colors from different authors applied in a differential sense have smaller uncertainties. (4) In the present models the dominant error for colors is probably the transformation from stellar atmospheric parameters to stellar colors. (5) Stellar B - V is difficult to model, and current spreads among different authors can reach 0.2 mag. (6) If known defects in the stellar nux library are corrected, the population model colors of this work in passbands redder than U would be accurate to roughly 0.03 mag in an absolute sense. These corrections are not made in the tables of model output.