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 Hβ. A model of the ionized region is constructed, using the photoionization 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 photoionization model allows 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 blackbody approximations. 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 Far-Infrared Absolute Spectrophotometer (FIRAS) spectrum taken by COBE. Because our line of sight is in the Galactic plane, the very cold component could be a starless core.
Digital Object Identifier (DOI)
van Hoof, Peter A. M.; Van de Steene, Griet C.; Beintema, Douwe A.; Martin, P. G.; Pottasch, Stuart R.; and Ferland, Gary J., "Properties of Dust Grains in Planetary Nebulae. I. The Ionized Region of NGC 6445" (2000). Physics and Astronomy Faculty Publications. 111.