Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
A. Gauger, Y.Y.Balega, P. Irrgang, R.
Osterbart and G. Weigelt:
High-resolution speckle masking
interferometry and radiative
transfer modeling
of the oxygen-rich AGB star AFGL 2290
Astronomy and Astrophysics 346, 505-519 (1999)
Abstract.
We present the first diffraction-limited speckle masking observations
of the oxygen-rich AGB star AFGL 2290. The speckle
interferograms were recorded with the Russian 6m SAO telescope. At
the wavelength 2.11µm a resolution of 75 milli-arcsec
(mas) was obtained. The reconstructed diffraction-limited image
reveals that the circumstellar dust shell (CDS) of AFGL 2290
is at least slightly non-spherical. The visibility function shows
that the stellar
contribution to the total 2.11µm flux is less than 40%, indicating a
rather large optical depth of the circumstellar
dust shell. The 2-dimensional Gaussian visibility fit yields a
diameter of AFGL 2290 at 2.11µm of
43mas x 51mas, which corresponds to a diameter of
42AU x 50AU for an adopted distance of 0.98kpc.
Our new observational results provide additional constraints on the
CDS of AFGL 2290, which supplement the information from the
spectral energy distribution (SED). To determine the structure and the
properties of the CDS we have performed radiative transfer
calculations for spherically symmetric dust shell models.
The observed SED approximately at phase 0.2 can be well reproduced at
all wavelengths by a model with Teff=2000K, a dust
temperature of 800K at the inner boundary, an optical depth
tau_v=100 and a radius for the single-sized grains of a=0.1µm.
However, the 2.11µm visibility of
the model does not match the observation.
Exploring the parameter space, we found that grain size is the key
parameter in achieving a fit of the observed visibility while retaining
the match of the SED, at least partially. Both the slope and the
curvature of the visibility strongly constrain the possible grain
radii. On the other hand, the SED at longer wavelengths, the silicate
feature in particular, determines the dust mass loss rate and, thereby,
restricts the possible optical depths of the model.
With a larger grain size of 0.16µm and a higher
tau_v=150, the observed visibility can be reproduced preserving
the match of the SED at longer wavelengths. Nevertheless, the model
shows a deficiency of flux at short wavelengths, which is attributed
to the model assumption of a spherically symmetric dust distribution,
whereas the actual structure of the CDS around AFGL 2290 is
in fact non-spherical.
Our study demonstrates the possible limitations of dust shell models
which are constrained solely by the spectral energy distribution, and
emphasizes the importance of high spatial resolution observations for
the determination of the structure and the properties of circumstellar
dust shells around evolved stars.
You can get this publication ...