Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
T. Blöcker, Y. Balega, K.-H. Hofmann,
J. Lichtenthäler, R. Osterbart
and G. Weigelt:
The rapidly evolving hypergiant IRC+10420:
High-resolution bispectrum speckle-interferometry
and dust-shell modelling
Astronomy and Astrophysics 348, 805-814 (1999)
Abstract.
The hypergiant IRC+10420 is a unique object for the
study of stellar evolution since it is the only object that is
believed to be witnessed in its rapid transition from the red
supergiant
stage to the Wolf-Rayet phase. Its effective temperature has increased
by
1000-2000K within only 20yr.
We present the first speckle observations
of IRC+10420 with 73mas resolution.
A diffraction-limited 2.11µm image was reconstructed
from 6m telescope speckle data using the
bispectrum speckle-interferometry method. The visibility function shows
that the dust shell contributes ~40% to the total flux and
the unresolved central object ~60%.
Radiative transfer calculations have been performed
to model both the spectral energy distribution and visibility function.
The grain sizes, a, were found to be in accordance with
a standard distribution function,
n(a) ~ a^(-3.5), with a ranging between
amin=0.005µm and amax=0.45µm.
The observed dust shell properties cannot be fitted by
single-shell models but seem to require multiple components.
At a certain distance we considered an enhancement over the assumed
1/r^x
density distribution.
The best model for both SED and visibility was
found for a dust shell with a dust temperature of 1000K at its inner
radius
of 69Rstar. At a distance of
308Rstar the density was enhanced by a factor of 40 and and its
density exponent was changed from x=2 to x=1.7.
The shell's intensity distribution was found to be ring-like.
The ring diameter
is equal to the inner diameter of the hot shell (69mas).
The diameter of the central star is ~1mas.
The assumption of a hotter inner shell of 1200K gives fits of almost
comparable quality but decreases the spatial
extension of both shells' inner boundaries
by ~30% (with x=1.5 in the outer shell).
The two-component model can be interpreted in terms of a termination of
an
enhanced mass-loss phase roughly 60 to 90 yr (for d=5kpc) ago.
The bolometric flux, Fbol, is
8.17x10^(-10)Wm^-2 corresponding to a central-star luminosity of L/Lsol
= 25462x(d/kpc)^2.
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