High-resolution bispectrum speckle interferometry and
two-dimensional radiative transfer modeling of the Red Rectangle
A.B. Men'shchikov, Y.Y. Balega, R. Osterbart and G. Weigelt
New Astronomy 3, 601-617 (1998)
Abstract.
We present the first diffraction-limited K-band image of the Red
Rectangle with 76mas resolution, an H-band image with 75mas resolution,
and an RG715 filter image (800nm wavelength) with 78mas resolution
(corresponding to 25 AU for a distance of 330 pc). The H and K
images were reconstructed from 6m telescope speckle data and the RG715
image from 2.2m telescope data using the speckle masking bispectrum method.
At all wavelengths the images show a compact, highly symmetric bipolar nebula,
suggesting a toroidal density distribution of the circumstellar material. No
direct light from the central binary can be seen as it is obscured by a dust
disk or circumbinary torus. Our first high-resolution H-K color image of
the nebula shows a broad red plateau of H-K approximately 2 mag, in the bright
inner regions.
The optical and near-infrared images and the available photometric continuum
observations in a wide range of ultraviolet to centimeter wavelengths enabled
us to model the Red Rectangle in detail using a two-dimensional
radiative transfer code. Our model matches both the high-resolution images
and the spectral energy distribution of this object very well, making the
following picture much more certain. The central close binary system with a
total luminosity of 3000 Lsun, is embedded in a very dense, compact
circumbinary torus which has an average number density of
n_H = 5x10^12 cm^-3, an outer radius of the
dense inner region of R = 30 AU (91mas), and a
r^-2 density distribution.
The full opening angle of the bipolar outflow cavities
in our model is 70 degrees.
By comparing the observed and theoretical images, we
derived an inclination angle of the torus to the line of sight of
(7 +/- 1) degrees.
The radiative transfer calculations show that the dust properties in the
Red Rectangle are spatially inhomogeneous. Our modeling confirms
that the idea of large grains in the long-lived disk around the
Red Rectangle (Jura et al. 1997) is quantitatively consistent with
the observations. In our models, unusually large, approximately
millimeter-sized grains dominate the emission of the compact, massive torus.
Models with smaller average grain sizes can possibly be found in future
studies, for instance, if it turns out that the radio spectrum is not mainly
caused by continuum dust emission. Therefore, the large grains suggested by
our models require further confirmation by both new observations and radiative
transfer calculations.
Assuming a dust-to-gas ratio rho_d/rho_g
of 0.005, the dense torus mass is 0.25 Msun. The model gives a lower limit
of 0.0018 Msun, for the mass of the large particles with an average radius
of 2mm, which produce a gray extinction of A=28mag, towards
the center. A much smaller mass of submicron--sized dust grains is presumably
located in the polar outflow cavities, their conical surface layers, and in
the outer low--density parts of the torus (where the density is proportional
to r^-4, in the
region of 30 AU < r < 2000 AU corresponding to 0.09 - 6 arcseconds).