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)
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 RG 715 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 RG 715 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 ≈ 2m, 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 L☉, is embedded in a very dense, compact circumbinary torus which has an average number density of nH ≈ 5 × 1012 cm-3, an outer radius of the dense inner region of R ≈ 30 AU (91mas), and a ρ∝r-2 density distribution. The full opening angle of the bipolar outflow cavities in our model is 70°. By comparing the observed and theoretical images, we derived an inclination angle of the torus to the line of sight of 7° ± 1°.
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 [ApJ, 474, 741]) 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 ρd/ρg of 0.005, the dense torus mass is 0.25 M☉. The model gives a lower limit of 0.0018 M☉, for the mass of the large particles with an average radius of 2mm, which produce a gray extinction of A ≈ 28m, 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 ρ∝r-4, in the region of 30 AU ≲ r ≲ 2000 AU corresponding to 0.09 - 6 arcseconds).
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