Properties of dust grains in planetary nebulae. I. The ionized region of NGC 6445

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. S everal processes have been proposed that either destroy the grains or remov e them from the ionized region. To test whether these processes are effecti ve, we study new infrared spectra of the evolved nebula NGC 6445. These dat a 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, usi ng the photoionization code CLOUDY 90.05. Based on this model, we show from depletions in the gas-phase elements that Little grain destruction can hav e occurred in the ionized region of NGC 6445. We also argue that dust-gas s eparation in the nebula is not plausible. The most likely conclusion is tha t grains are residing inside the ionized region of NGC 6445 and that the lo w temperature and flux of the grain emission are caused by the low luminosi ty of the central star and the low optical depth of the grains. This implie s that the bulk of the silicon-bearing grains in this nebula were able to s urvive exposure to hard-UV photons for at least several thousands of years, contradicting previously published results. A comparison between optical a nd infrared diagnostic line ratios gives a marginal indication for the pres ence of a t(2) effect in the nebula. However, the evidence is not convincin g and the differences could also be explained by uncertainties in the absol ute flux calibration of the spectra, the aperture corrections that have bee n applied, or the collisional cross sections. The photoionization model all ows an accurate determination of the central star temperature based on mode l atmospheres. The resulting value of 184 kK is in good agreement with the average of all published Zanstra temperatures based on black-body approxima tions. 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 Sp ectrophotometer (FIRAS) spectrum taken by COBE. Because our line of sight i s in the Galactic plane, the very cold component could be a starless core.