Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
B. Yudin, Y. Balega, T. Blöcker, K.-H.
Hofmann, D. Schertl, and
G. Weigelt:
Speckle interferometry and radiative
transfer modelling of the Wolf-Rayet star WR 118
Astronomy and Astrophysics 379, 229-234 (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 2.13µm speckle interferometric
observations of WR 118 with 73 mas 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 (17 +/- 1)mas),
the temperature of the smallest grains and the dust shell density are
(1750+/-100)K and (1 +/- 0.2) 10-19g/cm^3, respectively. The
dust formation rate is found to be (1.3 +/- 0.5) 10-7
Msol/yr, assuming vwind = 1200km/s.
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