SIMBAD references

We present three new images of the young, carbon rich planetary nebula, IC 418: 11.3µm dust emission, a [Ne II]12.8µm line emission and 6cm free-free continuum. All three images show different morphologies. In order to investigate these spatial differences and the mechanisms of dust emission in IC 418, we compare our data to two radiative transfer models with different radial density distributions. Model 1 has a thin shell that drops off as r^–3, resulting in a high density ionized region surrounded by an ionized halo. While, model 2 has a thicker shell that drops off as r^–2 resulting in the ionization front stopping in the thick shell and a high density ionized region immediately surrounded by a neutral shell. Both models use a mixture of silicon carbide (SiC) and amorphous carbon (AC) dust grains with dust to gas ratios of ∼2x10^–5 and ∼6x10^–4, respectively and the standard power law distribution in sizes (a^–3.5; 0.005µm<a<0.25µm). Both models reproduce our mid-IR images and mid-IR spectra well suggesting that, while IC 418 has emission features attributed to polycyclic aromatic hydrocarbons (PAHs), the SiC feature and not the PAH feature probably dominates the 11.3µm emission. Comparison of our images with broad-band J, H and K images of IC 418 by Hora et al. (1993), suggest that dust emission processes may contribute more near-IR emission than previously thought and that this near-IR "dust" emission is contained within the ionized gas region. Both models reproduce the observed spatial distributions of the 11.3 µm, [Ne ii], and 6 cm emissions which differ because they have different radial excitation gradients in the nebula. However, model 1 better explains all of the many previous observations of IC 418 at different wavelengths. We suggest a three layer onion model for IC 418: a ∼6" radius high density ionized region surrounded by a ∼20" radius low density ionized halo, enclosed by a completely photodissociated neutral halo extended beyond a ∼80" radius. This structure may suggest a mass loss history for IC 418 in which its progenitor AGB star experienced a superwind ({dot}(M)∼4x10^–5M_☉/yr, v∼11km/s) just before departing the AGB.