On the nature of interstellar grains and the interpretation of the IRAS background radiation

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Abstract
The interstellar extinction curve has been accurately fitted with a simple composite model for interstellar grains consisting of small (≤0.01 and 0.03 μm radius) grains of graphite and silicate, and larger (∼0.1 μm) grains of amorphous carbon and amorphous silicate. This model is also consistent with observations of circumstellar dust shells around late-type stars and accounts naturally for the far-infrared spectra of hot-centred clouds associated with newly formed massive stars. Essentially all the interstellar silicon and 35 per cent of the interstellar carbon would be locked up in grains. The interstellar ‘cirrus’ found by IRAS can be identified as due to the small grains, primarily the carbon. It is found necessary to assume that the energy-density of the interstellar radiation field in the vicinity of the high-latitude cirrus clouds is approximately twice that in the solar neighbourhood. Emission from interstellar dust is likely to have a dominant effect on surveys and background studies in the wavelength range 100 μm–1 mm. An upper limit of 0.8 MJy/sr can be set to the contribution of interstellar grains to the 100 μm background intensity, outside obvious regions of cirrus and neutral hydrogen. This leaves a residual background of 5–6 MJy/sr unaccounted for by interplanetary or interstellar dust.

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