Астрофизический семинар ИНАСАН № 50 (11 июня 2003 г., 11:00)Опубликовано: 11/06/2003
Докладчик: Меньшиков А. (Институт радиоастрономии им. Макса Планка, Бонн)
Название доклада: “Детальное двумерное моделирование переноса излучения в околозведных оболочках вокруг звезд на поздних стадиях эволюции (IRC+10216, Red Rectangle)”
Краткое содержание доклада:
IRC+10216 in Action: Present Episode of Intense Mass-Loss Reconstructed by Two-Dimensional Radiative Transfer Modeling
Alexander Men’shchikov, Karl-Heinz Hofmann, & Gerd Weigelt
We present two-dimensional (2D) radiative transfer modeling of IRC+10216 at selected moments of its evolution in 1995-2001, which correspond to three epochs of our series of 8 near-infrared speckle images (Osterbart et al. 2000, Weigelt et al. 2002). The high-resolution images obtained over the last 5.4 years revealed the dynamic evolution of the subarcsecond dusty environment of IRC+10216 and our recent time-independent 2D radiative transfer modeling reconstructed its physical properties at the single epoch of January 1997 (Men’shchikov et al. 2001). Having documented the complex changes in the innermost bipolar shell of the carbon star, we incorporate the evolutionary constraints into our new modeling to understand the physical reasons for the observed changes. The new calculations show that our previous static model is consistent with the brightness variations seen in the near-infrared images, implying that during the last 50 years, we have been witnessing an episode of a steadily increasing mass loss from the central star, from Mdot ~ 10-5 Msun/yr to the rate of Mdot ~ 3×10-4Msun/yr in 2001. The rapid increase of the mass loss of IRC+10216 and continuing time-dependent dust formation and destruction caused the observed displacement of the initially faint components C and D and of the bright cavity A from the star which has almost disappeared in our images in 2001. Increasing dust optical depths are causing strong backwarming that leads to higher temperatures in the dust formation zone, displacing the latter outward with a velocity vT ~ 27 km/s due to the evaporation of the recently formed dust grains. This self-regulating shift of the dust density peak in the bipolar shell mimics a rapid radial expansion, whereas the actual outflow has probably a lower speed v < vinf ~ 15 km/s. The model predicts that the star will remain obscured until Mdot starts to drop back to lower values in the dust formation zone; in a few years from that moment, we could be witnessing the star reappearing.
Properties of the Close Binary and Circumbinary Torus of the Red Rectangle
Alexander Men’shchikov, Dieter Schertl, Peter Tuthill, Gerd Weigelt, & Lev Yungelson
New diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1–3.3 mkm with angular resolutions of 44–68 mas (Tuthill et al. 2002) and previous speckle images at 0.7–2.2 mkm (Osterbart et al. 1997, Men’shchikov et al. 1998) revealed well-resolved bright bipolar outflow lobes and long X-shaped spikes originating deep inside the outflow cavities. This set of high-resolution images stimulated us to reanalyze all infrared observations of the Red Rectangle using our two-dimensional radiative transfer code. The high-resolution images imply a geometrically and optically thick torus-like density distribution with bipolar conical cavities and are inconsistent with the flat disk geometry frequently used to visualize bipolar nebulae. The new detailed modeling, together with estimates of the interstellar extinction in the direction of the Red Rectangle enabled us to more accurately determine one of the key parameters, the distance D = 710 pc with model uncertainties of 70 pc, which is twice as far as the commonly used estimate of 330 pc. The central binary is surrounded by a compact, massive (M = 1.2 Msun), very dense dusty torus with hydrogen densities reaching nH = 2.5 x 1012 cm-3 (dust-to-gas mass ratio rhod/rho ~ 0.01). The model implies that most of the dust mass in the dense torus is in very large particles and, on scales of more than an arcsecond, the polar outflow regions are denser than the surrounding medium. The bright component of the spectroscopic binary HD 44179 is a post-AGB star with mass M* = 0.57 Msun, luminosity L* = 6000 Lsun, and effective temperature T* = 7750 K. Based on the orbital elements of the binary, we identify its invisible component with a helium white dwarf with Mwd ~ 0.35 Msun, Lwd ~ 100 Lsun, and Twd ~ 6 x 104 K. The hot white dwarf ionizes the low-density bipolar outflow cavities inside the dense torus, producing a small HII region observed at radio wavelengths. We propose an evolutionary scenario for the formation of the Red Rectangle nebula, in which the binary initially had 2.3 and 1.9 Msun components at a separation of 130 Rsun. The nebula was formed in the ejection of a common envelope after Roche lobe overflow by the present post-AGB star.