Three-dimensional dust radiative-transfer models: the Pinwheel Nebula of WR 104

We present radiative-transfer modelling of the dusty spiral Pinwheel Nebula observed around the Wolf-Rayet/OB-star binary WR 104. The models are based on the three-dimensional radiative-transfer code TORUS, modified to include an adaptive mesh that allows us to adequately resolve both the inner spiral turns (subau scales) and the outer regions of the nebula (distances of 10(4) au from the central source). The spiral model provides a good fit to both the spectral energy distribution and Keck aperture masking interferometry, reproducing both the maximum entropy recovered images and the visibility curves. We deduce a dust creation rate of 8+/-1x10(-7) M-circle dot yr(-1), corresponding to approximately 2 per cent by mass of the carbon produced by the Wolf-Rayet star. Simultaneous modelling of the imaging and spectral data enables us to constrain both the opening angle of the wind-wind collision interface and the dust grain size. We conclude that the dust grains in the inner part of the Pinwheel Nebula are small (similar to100 Angstrom), in agreement with theoretical predictions, although we cannot rule out the presence of larger grains (similar to1 mum) further from the central binary. The opening angle of the wind-wind collision interface appears to be about 40degrees, in broad agreement with the wind parameters estimated for the central binary. We discuss the success and deficiencies of the model, and the likely benefits of applying similar techniques to the more complex nebulae observed around other WR/O star binaries.