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

Free access article

Issue		A&A Volume 379, Number 1, November III 2001


Page(s)		229 - 234
Section		Formation, structure and evolution of stars
DOI		10.1051/0004-6361:20011289

A&A 379, 229-234 (2001)
DOI: 10.1051/0004-6361:20011289

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

B. Yudin¹, Y. Balega², T. Blöcker³, K.-H. Hofmann³, D. Schertl³ and G. Weigelt³

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

(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 $\mu$ 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 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 $\mu$ m, and our 2.13 $\mu$ m 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 $\mu$ m and 0.38 $\mu$ 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 $n(a) \sim a^{-3}$ , with a ranging between 0.005 $\mu$ m and 0.6 $\mu$ m. At the inner boundary of the modelled dust shell (angular diameter $\Theta_{\rm in} = (17 \pm 1)$ mas), the temperature of the smallest grains and the dust shell density are 1750 K $\pm$ 100 K and $(1 \pm 0.2) \times 10^{-19}$ g/cm³, respectively. The dust formation rate is found to be $(1.3 \pm 0.5) \times 10^{-7}$ $M_{\odot}$ /yr, assuming $V_{\rm wind} = 1200$ km s^-1.

Key words: techniques: image processing -- circumstellar matter -- stars: individual: WR 118 -- stars: mass-loss -- stars: Wolf-Rayet -- infrared: stars

Offprint request: B. Yudin, yudin@sai.msu.ru

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