We present radiative-transfer modelling of the dusty spiral Pinwheel Nebula observed around the Wolf-Rayet/OB-star binary WR104 .
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 ( sub-AU 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 \pm 1 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } , corresponding to approximately 2 % 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 ( \sim 100 Å ) , in agreement with theoretical predictions , although we can not rule out the presence of larger grains ( \sim 1 µm ) further from the central binary .
The opening angle of the wind-wind collision interface appears to be about 40 ^ { \circ } , 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 the more complex nebulae observed around other WR/O star binaries .