5.1 Models
The evolutionary population synthesis (EPS) is the technique to model
the spectrophotometric properties of stellar populations, that uses
the knowledge of stellar evolution. This approach was pioneered by
B. Tinsley in a series of fundamental papers, that provide the basic
concepts still used in present-day computations. The models are used
to determine ages, element abundances, stellar masses, stellar mass
functions, etc., of those stellar populations that are not resolvable
in single stars, like galaxies and extra-galactic globular
clusters.
Evolutionary population synthesis models for a wide range of
metallicities, ages, star formation histories, initial mass functions,
and Horizontal Branch morphologies, including blue morphologies at
high metallicity, are computed. The model output comprises spectral
energy distributions, colours, stellar M/L ratios, bolometric
corrections, and near-infrared spectral line indices. The energetics
of the post Main Sequence evolutionary phases are evaluated with the
fuel consumption theorem.
The models are calibrated with globular cluster data from the Milky
Way for old ages, and the Magellanic Clouds plus the merger remnant
galaxy NGC~7252, both for young ages of ∼ 0.1-2 Gyr, in a large
wavelength range from the U-band to the K-band. Particular
emphasis is put on the contribution from the Thermally-Pulsing
Asymptotic Giant Branch phase. We show that this evolutionary phase is
crucial for the modelling of young stellar populations by the direct
comparison with observed spectral energy distributions of Magellanic
Clouds clusters, which are characterised by relatively high fluxes
both blueward and redward the V-band.
In a further set of calculations, we provide the whole set of Lick
indices from Hδ and CN_1 to TiO_2 in the wavelength-range
4000<λ< 6500 A of Simple Stellar Population models
with, for the first time, variable element abundance ratios,
[α/Fe]=0.0, 0.3, 0.5,
[α/Ca]=-0.1, 0.0, 0.2, 0.5, and
[α/N]=-0.5, 0.0. The models cover ages between 0.1 and
15 Gyr, metallicities between 1/200 and 3.5 solar. The impact from
the element abundance changes on the absorption-line indices are taken
from Korn, Maraston, Thomas (A&A, submitted), using an extension of
the method introduced by Trager et al. (2000). Our models are free
from the intrinsic α/Fe bias that was imposed by the Milky Way
template stars up to now, hence they reflect well-defined α/Fe
ratios at all metallicities. The models are calibrated with Milky Way
globular clusters for which metallicities and α/Fe ratios are
known from independent spectroscopy of individual stars. The
metallicities that we derive from the Lick indices Mgb and Fe5270 are
in excellent agreement with the metallicity scale by Zinn and West
(1984).
last update:
10/2004, editor of this page: Roberto Philip Saglia
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