Modeling dust emission in PN IC 418

Abstract

We investigated the infrared (IR) dust emission from PN IC 418, using a detailed model controlled by a previous determination of the stellar properties and the characteristics of the photoionized nebula, keeping as free parameters the dust types, amounts, and distributions relative to the distance of the central star. The model includes the ionized region and the neutral region beyond the recombination front (photodissociation region, or PDR), where the [O I] and [C II] IR lines are formed. We succeeded in reproducing the observed infrared emission from 2 to 200 μm. The global energy budget is fitted by summing up contributions from big grains of amorphous carbon located in the neutral region and small graphite grains located in the ionized region (closer to the central star). Two emission features seen at 11.5 and 30 μm are also reproduced by assuming them to be due to silicon carbide (SiC) and magnesium and iron sulfides (MgxFe1−xS), respectively. For this, we needed to consider ellipsoidal shapes for the grains to reproduce the wavelength distribution of the features. Some elements are depleted in the gaseous phase: Mg, Si, and S have sub-solar abundances (−0.5 dex below solar by mass), while the abundance of C + N + O + Ne by mass is close to solar. Adding the abundances of the elements present in the dusty and gaseous forms leads to values closer to but not higher than solar, confirming that the identification of the feature carriers is plausible. Iron is strongly depleted (3 dex below solar) and the small amount present in dust in our model is far from being enough to recover the solar value. A remaining feature is found as a residue of the fitting process, between 12 and 25 μm, for which we do not have identification.