The circumstellar dust shell of IRC + 10216

The energy distribution of the bright infrared object IRC + 10216 is discussed in terms of an extended, optically thick dust shell. It is shown that the general spectral and spatial characteristics of the source are well described by a spherical graphite shell, although there is evidence for some departure from spherical symmetry in the dust distribution. There is also evidence that the optical parameters of the grains may differ substantially from the graphite data employed here. The maximum grain temperature in the dust model is significantly below the expected condensation temperature of graphite, and possible reasons for this are discussed. The observed 10– 13 μ m feature can be understood in terms of a graphite shell containing 1–2 per cent by number of spheroidal silicon carbide (SiC) particles, whose shape distribution is weighted in favour of prolate grains. Consideration of the SiC2 molecule in relation to the SiC dust suggests the possibility of a correlation between the blue–green Merrill–Sanford bands and the 10–13 μ m emission feature.