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Planetary Nebulae and Their Role in the Universe
IAU Symposium 209, 2002
M. Dopita, S. Kwok and R.S. Sutherland, eds.
Diraction-limited speckle interferometry and modeling of
the circumstellar envelope of R CrB at maximum and
minimum light
K. Ohnaka, T. Blocker, K.-H. Hofmann, N. R. Ikhsanov, G. Weigelt
Max-Planck-Institut fur Radioastronomie, 53121 Bonn, Germany
Y. Balega
Special Astrophysical Observatory, Nizhnij Arkhyz, 35147, Russia
V. I. Shenavrin, B. F. Yudin
Sternberg Astronomical Institute, 119899 Moscow, Russia
Y. S. Emov
Crimean Astrophysical Observatory, Nauchny 98409, Ukraine
We present the rst speckle interferometric observations of R CrB, the prototype
of a class of peculiar stars which undergo irregular declines in their visible light
curves. The observations were carried out with the 6 m telescope at the Special
Astrophysical Observatory near maximum light (V = 7, 1996 Oct. 1) and at
minimum light (V = 10:61, 1999 Sep. 28). A spatial resolution of 75 mas was
achieved in the K-band. The dust shell around R CrB is partially resolved,
and the visibility is approximately 0.8 at a spatial frequency of 10 cycles/arcsec.
The two-dimensional power spectra obtained at both epochs do not show any
signicant deviation from circular symmetry. The visibility function and spectral
energy distribution obtained near maximum light can be simultaneously tted
with a model consisting of the central star and an optically thin dust shell with
density proportional to r 2 and amorphous carbon as its constituent. The inner
boundary of the shell is found to be 82 R ?
(19 mas) with a temperature of
920 K near maximum light. However, this simple model fails to simultaneously
reproduce the visibility and spectral energy distribution obtained at minimum
light. We show that this discrepancy can be attributed to thermal emission from
a newly formed optically thick dust cloud. Simultaneous ts of the observed SED
and visibility with models including a thermally emitting dust cloud suggest the
presence of a newly formed dust cloud as hot as 1200 K with a radius of 4 { 5 R ? ,
in addition to an optically thin dust shell whose inner boundary is  170 R ?
with a temperature of  690 K. Alternatively the discrepancy can be attributed
to an unusual extinction curve of dust grains in the obscuring cloud which was
present in front of the star at minimum light. The details of the observations
and modeling are described in Ohnaka et al. (2001, A&A 380, 212).
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