The circumstellar dust shell ?

A spherically symmetric dust radiative transfer code is used to model the circumstellar dust shell around IRC +10 216. Compared to numerous previous models a much larger body of observational data is used as constraints; the spectral energy distribution between 0.5 and 60000 µm, 2-4 µm and 8-23 µm spectra, optical, far-infrared and centimeter sizes and interferometric visibility curves between 1.6 and 11.2 µm are used to constrain the model. The most important result is that in order to fit the visibility curve at 2.2 µm and the size of the shell in the optical, scattering has to be invoked. The strong dependence of the scattering coefficient on grain size allows one to derive a mean grain size of 0.16 ± 0.01 µm. For a model with a r −2 density distribution a dust mass loss rate of 8.1 ± 0.7 × 10 −7 D (kpc) M yr −1 (adopting v ∞ = 17.5 km s −1 and a dust opacity κ 60µm = 68 cm 2 g −1), a luminosity at maximum light of 823 ± 40 × 10 3 (D (kpc)) 2 L , an inner dust radius of r c = 4.5 ± 0.5 R , an inner dust temperature of T c = 1075 ± 50 K and an effective temperature of T eff = 2000 ± 100 K are derived (all 1σ error bars). It is found that a r −2 dust density law in the inner part of the shell gives a slightly better fit than the physically more realistic case of a steeper law where the effect of the increasing dust velocity with radius is taken into account. It is suggested that the dust-togas ratio also increases with radius and that therefore the net effect on the dust density distribution may be small. Previous suggestions that the mass loss rate was higher in the past are confirmed. The principle argument is that with an r −2 model the calculated far-infrared sizes are smaller than observed. A good fit is obtained with a dust mass loss rate of 8.1 × 10 −7 D (kpc) M yr −1 for r < 123 and 7.3 × 10 −6 D (kpc) M yr −1 for r > 123 (assuming that v ∞ and the dust opacity do not change with time). An alternative model with an exponentially decreasing mass loss rate can be excluded. The presently available …