Grain size distributions and photo-electric heating in ionized media
van Hoof P.A.M., Weingartner J.C., Martin P.G., Volk K., Ferland G.J.
published in: MNRAS, 350, 1330 (2004)
Ever since the pioneering study of Spitzer, it has been widely recognized that
grains play an important role in the heating and cooling of photo-ionized
environments. This includes the diffuse ISM, as well as H II regions,
planetary nebulae, and photo-dissociation regions. A detailed code is
necessary to model grains in a photo-ionized medium since the interactions of
grains with their environment include a host of microphysical processes. In
this paper we will use the spectral synthesis code Cloudy for this purpose. A
comprehensive upgrade of the grain model has been recently incorporated into
Cloudy. One of these upgrades is the newly developed hybrid grain charge
model. This model allows discrete charge states of very small grains to be
modelled accurately while simultaneously avoiding the overhead of fully
resolving the charge distribution of large grains, thus making the model both
accurate and computationally efficient. A comprehensive comparison with the
fully resolved charge state models of Weingartner & Draine (2001) shows that
the agreement is very satisfactory for realistic size distributions. The
effect of the grain size distribution on the line emission from photo-ionized
regions is studied by taking standard models for an H II region and a
planetary nebula and adding a dust component to the models with varying grain
size distributions. A comparison of the models shows that varying the size
distribution has a dramatic effect on the emitted spectrum. The strongest
enhancement is always found in optical/UV lines of the highest ionization
stages present in the spectrum (with factors up to 2.5 to 4), while the
strongest decrease is typically found in optical/UV lines of low ionization
lines or infrared fine-structure lines of low/intermediate ionization stages
(with reductions up to 10 to 25 percent). Changing the grain size distribution also
affects the ionization balance, and can affect resonance lines which are
very sensitive to changes in the background opacity. All these results clearly
demonstrate that the grain size distribution is an important parameter in
photo-ionization models.
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Peter van Hoof
Royal Observatory of Belgium
Ringlaan 3
1180 Brussel
Belgium
email: p DOT vanhoof AT oma DOT be
