Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
P. Irrgang, Y. Balega, A. Gauger,
R. Osterbart, G. Schniggenberg and G. Weigelt:
Speckle Masking Imaging and Radiative
Transfer Modeling of
the Oxygen-rich Dust Shells of AFGL 2290 and CIT 3
Abstract (poster) for
IAU Symp. 191, Asymptotic Giant Branch Stars, Aug 27th - Sep 1rst,
1998,
Montpellier, France
Abstract.
The extreme mass loss suffered by stars on the asymptotic giant branch
(AGB) substantially affects the appearance of these objects, which is
dominated by the development of opaque, massive, circumstellar dust
shells (CDS), and even more important, it determines the final
evolution to the planetary nebula stage.
High spatial resolution observations directly provide information on
important properties of CDS around AGB stars, such as the dimensions
and geometry of the shell, and thereby contribute strong constraints
for the modeling of the mechanisms and processes determining these
circumstellar environments.
We present diffraction-limited speckle masking
observations
of the oxygen-rich AGB stars AFGL 2290 (OH 39.7+1.5) at 2.2 micron
and of CIT 3 (OH 128.6-50.1) at 1.65 micron and
2.2 micron. The speckle interferograms were obtained with the
SAO 6 m telescope, and we achieved the diffraction-limited
resolutions of 56 mas and 76 mas at 1.65 micron and 2.2 micron,
respectively.
The CDS of AFGL 2290 is partially resolved and found to be
slightly
asymmetric with a mean Gaussian FWHM diameter of 42 mas.
The azimuthally averaged visibility yields an upper limit of
25% for the stellar contribution to the 2.2 micron
flux, suggesting a rather high optical depth. We have
performed detailed radiative transfer calculations assuming a
spherically symmetric dust shell, and found that such models reproduce
either the observed spectral energy distribution, or the measured
visibility of AFGL 2290, but not both simultaneously. We interpret
this behaviour as being due to a non-spherical dust distribution
supporting the evidence from the speckle masking image reconstruction.
The speckle imaging results for CIT 3 indicate a more
structured CDS
compared to AFGL 2290. The azimuthally averaged visibilities can only
be well fitted with a two-component model consisting of a partially
resolved smaller component and a fully resolved nebulosity, which is
several times more extended. The smaller component represents the
hot innermost region of the CDS, whereas the presence of the extended
component might point to a change of physical properties in the
outflow.
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