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

¹ Sternberg Astronomical Institute, Universitetskii pr. 13, 119899 Moscow, Russia
² Special Astrophysical Observatory, Nizhnij Arkhyz, Zelenchuk region, Karachai-Cherkesia, 35147, Russia e-mail: balega@sao.ru
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: bloecker@mpifr-bonn.mpg.de; hofmann@mpifr-bonn.mpg.de; schertl@mpifr-bonn.mpg.de; weigelt@mpifr-bonn.mpg.de

Corresponding author: B. Yudin, yudin@sai.msu.ru

Received: 16 July 2001
Accepted: 7 September 2001

Abstract

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 μm 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 ~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 0.5-25 μm, and our 2.13 μm 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 μm and 0.38 μm, and a mass fraction of the large grains greater than 65% . Alternatively, a good match can be obtained with the grain size distribution function , with a ranging between 0.005 μm and 0.6 μm. At the inner boundary of the modelled dust shell (angular diameter mas), the temperature of the smallest grains and the dust shell density are 1750 K ± 100 K and g/cm³, respectively. The dust formation rate is found to be /yr, assuming km s^-1.