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

Pam. Van Hoof et al., Properties of dust grains in planetary nebulae. I. The ionized region of NGC 6445, ASTROPHYS J, 532(1), 2000, pp. 384-399
Citations number
Categorie Soggetti
Space Sciences
Journal title
ISSN journal
0004-637X → ACNP
Year of publication
384 - 399
SICI code
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.