Since the earliest stages of massive star formation are generally
heavily enshrouded by dust, infrared observations are essential to
detect the youngest stellar populations and provide various diagnostic
features to study the properties of the interstellar matter (ISM) and
the underlying stars. We modeled the age evolution of
near- and mid-infrared spectral features during the youngest stages of
star formation (younger than 6 Myr) as a function of metallicity,
nebular density, and ionization parameter. The results of this model
grid are available in the table below.
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Recently, mid-infrared instruments have become available on several
large ground-based telescopes, resulting in data sets with
unprecedented spatial resolution at these long wavelengths. The main
motivation for our study was to investigate the spectral features
observable from the ground (within the N- and Q-band atmospheric
windows; 8 - 13 and 16.5 - 24.5 μm respectively). So,
complementary to the more commonly used mid-infrared line ratios,
e.g. [NeIII]/[NeII] and [SIV]/[SIII], the behaviour of
'ground-based-only' diagnostics like [SIV]/[ArIII] and [SIII]/[NeII]
can be found in this online data set.
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We used Starburst99
to create the input SEDs for our models; stellar clusters of a million Msun,
formed in a single burst, with a Salpeter IMF between 0.1 and 100
Msun, and metallicities of Z=0.4, 1, and 2 Zsun. Nebular properties
were varied, with a density of 1e2, 1e3, 1e4, 1e5, and 1e6 cm-3, and
ionization parameters between 2e7 and 8e8 cm s-1.
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The model grid is explained in detail in:
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Snijders, Kewley & Van der Werf, 2007, submitted to ApJ.
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The data available for downloading include the results for every
nebular density, ionization parameter and metallicity, also the ones
not shown in the figures in the paper. For each metallicity a
tar-package can be downloaded, each holding three tables:
- the age evolution of various emission line ratios -
[NeIII]/[NeII], [SIV]/SIII], [SIV]/[ArIII], [SIII]/[NeII], and HeI/HI
- as a function of density and ionization parameter
- the values for
[SIII]33.5μm/[SIII]18.7μm as a function of
density
- the equivalenth width of the hydrogen recombination line
Brγ as a function of age.
The tables are given both in ascii and ps.
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