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, CIV, and NV and by the stellar continuum are studied. The depletion of refractory elements and the attenuation of resonance lines in this nebula are discussed. (author)