MID-IR AND RADIO IMAGES OF IC-418 - DUST IN A YOUNG PLANETARY-NEBULA

We present three new images of the young, carbon rich planetary nebula , IC 418: 11.3 mu m dust emission, a [Ne II] 12.8 mu m line emission a nd 6 cm free-free continuum. All three images show different morpholog ies. In order to investigate these spatial differences and the mechani sms of dust emission in IC 418, we compare our data to two radiative t ransfer models with different radial density distributions. Model 1 ha s a thin shell that drops off as r(-3), resulting in a high density io nized region surrounded by an ionized halo. While, model 2 has a thick er shell that drops off as r(-2) resulting in the ionization front sto pping in the thick shell and a high density ionized region immediately surrounded by a neutral shell. Both models use a mixture of silicon c arbide (SiC) and amorphous carbon (AC) dust grains with dust to gas ra tios of similar to 2 x 10(-5) and similar to 6 x 10(-4), respectively and the standard power law distribution in sizes (a(-3.5); 0.005 mu m < a < 0.25 mu m). Both models reproduce our mid-IR images and mid-IR s pectra well suggesting that, while IC 418 has emission features attrib uted to polycyclic aromatic hydrocarbons (PAHs), the SiC feature and n ot the PAH feature probably dominates the 11.3 mu m emission. Comparis on of our images with broad-band J, H and K images of IC 418 by Hora e t 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 re produce the observed spatial distributions of the 11.3 mu m, [Ne II], and 6 cm emissions which differ because they have different radial exc itation 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 similar to 6'' radi us high density ionized region surrounded by a similar to 20'' radius low density ionized halo, enclosed by a completely photodissociated ne utral halo extended beyond a similar to 80'' radius. This structure ma y suggest a mass loss history for IC 418 in which its progenitor AGE s tar experienced a superwind (M over dot similar to 4 x 10(-5) M. yr(-1 ), v similar to 11 km s(-1)) just before departing the AGB.