Comprehensive Stellar Population Models and the Disentanglement of Age and Metallicity Effects
Abstract
The construction of detailed models for intermediate and old stellar populations is described. Input parameters include metallicity (-2 less than (Fe/H) less than 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 approximately equals 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 flux 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.
- Publication:
-
The Astrophysical Journal Supplement Series
- Pub Date:
- November 1994
- DOI:
- 10.1086/192096
- Bibcode:
- 1994ApJS...95..107W
- Keywords:
-
- Galactic Evolution;
- Globular Clusters;
- Hertzsprung-Russell Diagram;
- Horizontal Branch Stars;
- Main Sequence Stars;
- Metallicity;
- Stellar Evolution;
- Stellar Models;
- Color-Magnitude Diagram;
- Spectral Energy Distribution;
- Stellar Atmospheres;
- Stellar Color;
- Stellar Composition;
- Stellar Interiors;
- Stellar Luminosity;
- Stellar Magnitude;
- Stellar Mass;
- Stellar Spectrophotometry;
- Astrophysics;
- GALAXIES: ABUNDANCES;
- GALAXIES: ELLIPTICAL AND LENTICULAR;
- CD;
- GALAXIES: STELLAR CONTENT;
- STARS: HERTZSPRUNG-RUSSELL DIAGRAM;
- STARS: EVOLUTION;
- STARS: FUNDAMENTAL PARAMETERS