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 rst diraction-limited 2.13 m 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 signicant deviation from circular symmetry. The visibility curve declines towards the diraction cut-o frequency 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 m, and our 2.13 m visibility function, assuming spherical symmetry of the dust shell. Both can be tted with a model containing double-sized grains (\small" and \large") with the radii of a =0 :05 ma nd 0.38 m, 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 ,w itha ranging between 0.005 ma nd 0.6m. At the inner boundary of the modelled dust shell (angular diameter in =( 171) mas), the temperature of the smallest grains and the dust shell density are 1750 K 100 K and (1 0:2) 10 19 g/cm 3 , respectively. The dust formation rate is found to be (1:3 0:5) 10 7 M/yr, assuming Vwind = 1200 km s 1 .