Thermal infrared emission by dust in the planetary nebula NGC 3918 : a model analysis of IRAS observations.

Models of the dust grains in the planetary nebula NGC 3918 are presented. The models, which are calculated for four grain materials - graphite, amorphous carbon, silicate, and iron - have a size distribution of particles based on that found for the diffuse interstellar medium. The infrared spectrum of the nebula - described mainly by IRAS photometry corrected for line emission - can be matched either with graphite grains with a size range of 0.04 - 0.30 μm, or with amorphous carbon grains having a size range 0.0005 - 0.25 μm. The implied depletions of gas-phase carbon are only 11 and 4 per cent, respectively. It is shown that iron grains cannot be the dominant dust material in planetary nebulae. The relative heating rates for dust by the resonance lines of Lyα, C IV, and N V and by the stellar continuum are studied. The depletion of refractory elements and the attenuation of resonance lines in this nebula are discussed.