Properties of Dust Grains in Planetary Nebulae
I. The Ionized Region of NGC 6445
P.A.M. van Hoof, G.C. Van de Steene, D.A. Beintema, P.G. Martin, S.R. Pottasch, G.J. Ferland
published in: ApJ 532, 384 (2000)
One of the factors influencing the spectral evolution of a planetary
nebula is the fate of the dust grains that are emitting the infrared
continuum. Several processes have been proposed that either destroy
the grains or remove them from the ionized region. To test whether
these processes are effective, we study new infrared spectra of the
evolved nebula NGC 6445. These data show that the thermal
emission from the grains is very cool and has a low flux compared to
H beta. A model of the ionized region is constructed, using the
photo-ionization code CLOUDY 90.05. Based on this model, we show from
depletions in the gas phase elements that little grain destruction can
have occurred in the ionized region of NGC 6445. We also argue
that dust-gas separation in the nebula is not plausible. The most
likely conclusion is that grains are residing inside the ionized
region of NGC 6445 and that the low temperature and flux of the
grain emission are caused by the low luminosity of the central star
and the low optical depth of the grains. This implies that the bulk of
the silicon-bearing grains in this nebula were able to survive
exposure to hard UV photons for at least several thousands of years,
contradicting previously published results.
A comparison between optical and infrared diagnostic line ratios gives
a marginal indication for the presence of a t2-effect in the
nebula. However, the evidence is not convincing and the differences
could also be explained by uncertainties in the absolute flux
calibration of the spectra, the aperture corrections that have been
applied or the collisional cross sections. The photo-ionization model
allowed an accurate determination of the central star temperature
based on model atmospheres. The resulting value of 184 kK is in
good agreement with the average of all published Zanstra temperatures
based on a blackbody approximation.
The off-source spectrum taken with LWS clearly shows the presence of a
warm cirrus component with a temperature of 24 K as well as a
very cold component with a temperature of 7 K. Since our
observation encompasses only a small region of the sky, it is not
clear how extended the 7 K component is and whether it
contributed significantly to the FIRAS spectrum taken by COBE. Because
our line of sight is in the galactic plane, the very cold component
could be a starless core.
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Peter van Hoof
Royal Observatory of Belgium
Ringlaan 3
1180 Brussel
Belgium
email: p DOT vanhoof AT oma DOT be
