Speckle interferometry and radiative transfer modelling of the Wolf-Rayet star WR 118

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 2.13 mum speckle interferometric observations of WR 118 with 73 mas resolution. The speckle interferograms were obtained with the 6 m 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 similar to0.66 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 0.5-25 mum, and our 2.13 mum 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 = 0.05 mum and 0.38 mum, 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) similar to a(-3), with a ranging between 0.005 mum and 0.6 mum. At the inner boundary of the modelled dust shell (angular diameter Theta (in) = (17 +/- 1) mas), the temperature of the smallest grains and the dust shell density are 1750 K +/- 100 K and (1 +/- 0.2) x 10(-19) g/cm(3), respectively. The dust formation rate is found to be (1.3 +/- 0.5) x 10(-7) M./yr, assuming V-wind = 1200 km s(-1).