I. 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 density distribution a dust mass loss rate of 8.1 ± 0.7 × 10 D (kpc) M yr (adopting v∞ = 17.5 km s and a dust opacity κ60μm = 68 cm g), a luminosity at maximum light of 823 ± 40 × 10 (D (kpc)) L , an inner dust radius of rc = 4.5 ± 0.5 R? , an inner dust temperature of Tc = 1075 ± 50 K and an effective temperature of Teff = 2000 ± 100 K are derived (all 1σ error bars). It is found that a r 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-to-gas 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 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 D (kpc) M yr for r < 123 and 7.3 × 10 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. Send offprint requests to: Martin Groenewegen (groen@mpagarching.mpg.de) ? Tables 1, 2, 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) ar via http://cdsweb.u-strasbg.fr/Abstract.htlm The presently available constraints are not sensitive to the dust density beyond ∼10. The total dust mass in the shell out to 10 is 1.0 (D (kpc)) M in the model with the non constant mass loss rate, and 0.13 (D (kpc)) M in the model with the constant mass loss rate. The slope of the dust opacity beyond ∼1000 μm (where no laboratory measurements are available) and the influence of free-free emission are investigated by comparing cmobservations to a newly developed radiative transfer code to calculate the emission from a central star surrounded by a shell where free-free emission is assumed to occur. It is found that in small apertures dust emission is negligible for wavelengths > ∼ 2 cm. Free-free emission is negligible for wavelengths < ∼0.5 cm. To account for the observed flux at 3300 μm the slope of the opacity is changed to Qλ ∼ λ for λ > 1000 μm. The free-free emission is found to be optically thin even at 6 cm. An ionization fraction of 7.8 × 10 is derived which, according to the Saha equation, corresponds to an electron temperature of about 2400 K. Although there are uncertainties in the free-free emission model this suggests that the free-free emission does not come from a chromosphere.