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Properties of the close binary and circumbinary torus of the Red Rectangle
New diffraction-limited speckle images of the RedRectangle in the wavelength range 2.1-3.3 mu m with angularresolutions of 44-68 mas (Tuthill et al. \cite{Tuthill_etal2002}) andprevious speckle images at 0.7-2.2 mu m (Osterbart et al.\cite{Osterbart_etal1997}; Men'shchikov et al.\cite{Men'shchikov_etal1998}) revealed well-resolved bright bipolaroutflow lobes and long X-shaped spikes originating deep inside theoutflow cavities. This set of high-resolution images stimulated us toreanalyze all infrared observations of the RedRectangle using our two-dimensional radiative transfer code.The high-resolution images imply a geometrically and optically thicktorus-like density distribution with bipolar conical cavities and areinconsistent with the flat disk geometry frequently used to visualizebipolar nebulae. The new detailed modeling, together with estimates ofthe interstellar extinction in the direction of the RedRectangle enabled us to more accurately determine one of thekey parameters, the distance D ~ 710 pc with model uncertainties of 70pc, which is twice as far as the commonly used estimate of 330 pc. Thecentral binary is surrounded by a compact, massive (M ~ 1.2Msun), very dense dusty torus with hydrogen densitiesreaching nH ~ 2.5 x 1012 cm-3(dust-to-gas mass ratio \rhod/\rho ~ 0.01). The model impliesthat most of the dust mass in the dense torus is in very large particlesand, on scales of more than an arcsecond, the polar outflow regions aredenser than the surrounding medium. The bright component of thespectroscopic binary HD 44179 is a post-AGB star withmass M\star ~ 0.57 Msun, luminosityL\star ~ 6000 Lsun, and effective temperatureT\star ~ 7750 K. Based on the orbital elements of the binary,we identify its invisible component with a helium white dwarf withMWD ~ 0.35 Msun, LWD ~ 100Lsun, and TWD ~ 6 x 104 K. The hotwhite dwarf ionizes the low-density bipolar outflow cavities inside thedense torus, producing a small H II region observed at radiowavelengths. We propose an evolutionary scenario for the formation ofthe Red Rectangle nebula, in which the binaryinitially had 2.3 and 1.9 Msun components at a separation of~ 130 Rsun. The nebula was formed in the ejection of a commonenvelope after Roche lobe overflow by the present post-AGB star.
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