2001A&A...379..229Y

object{WR 118} is a highly evolved Wolf-Rayet star of the WC10 subtype surrounded by a permanent dust shell absorbing and re-emitting in the infrared a considerable fraction of the stellar luminosity. We present the first diffraction-limited 2.13µm speckle interferometric observations of WR 118 with 73mas resolution. The speckle interferograms were obtained with the 6m telescope at the Special Astrophysical Observatory. The two-dimensional visibility function of the object does not show any significant deviation from circular symmetry. The visibility curve declines towards the diffraction cut-off frequency to ∼0.66 and can be approximated by a linear function. Radiative transfer calculations have been carried out to model the spectral energy distribution, given in the range of 0.5-25µm, and our 2.13µm visibility function, assuming spherical symmetry of the dust shell. Both can be fitted with a model containing double-sized grains (``small'' and ``large'') with the radii of a=0.05µm and 0.38µm, and a mass fraction of the large grains greater than 65%. Alternatively, a good match can be obtained with the grain size distribution function n(a)∼a^–3, with a ranging between 0.005µm and 0.6µm. At the inner boundary of the modelled dust shell (angular diameter Θ_in=(17±1)mas), the temperature of the smallest grains and the dust shell density are 1750K±100K and (1±0.2)x10^–19g/cm³, respectively. The dust formation rate is found to be (1.3 ±0.5)x10^–7M_☉/yr, assuming V_wind=1200km/s.