Structure and physical properties of the rapidly evolving dusty envelope of IRC + 10216 reconstructed by detailed two-dimensional radiative transfer modeling

¹ Stockholm Observatory, 133 36 Saltsjöbaden, Sweden
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: bloecker@mpifr-bonn.mpg.de; osterbart@mpifr-bonn.mpg.de; weigelt@mpifr-bonn.mpg.de
³ Special Astrophysical Observatory, Nizhnij Arkhyz, 357147 Karachaevo-Cherkesia, Russia e-mail: balega@sao.ru

Corresponding author: A. B. Men'shchikov, sasha@astro.su.se

Received: 13 July 2000
Accepted: 12 December 2000

Abstract

We present the first detailed, two-dimensional radiative transfer model of the dusty envelope around the carbon star IRC + 10216. Our goal was to find a self-consistent model of the star and its envelope which takes into account as many observational constraints as possible. The model reproduces very well the entire beam-matched spectral energy distribution of IRC + 10216 from optical to centimeter wavelengths (at several phases of stellar luminosity), observed intensity profiles of the object at 1.25, 2.2, 10.5, 50, 100 m, and 1.3 mm, a 10.5 m lunar occultation intensity profile, our high-resolution , K, and bispectrum speckle-interferometry images, and visibilities in , and N bands. For the adopted distance of 130 pc, the model of IRC + 10216 implies that the object changes its luminosity between 13 000 and 5200 , its effective temperature between 2800 and 2500 K, and its radius between 500 and 390 . There is a dense non-spherical dust shell around the star, with outflow cavities at position angle PA20°. The southern cavity with a full opening angle of 36° is tilted toward us by 40° from the plane of sky, causing the observed bipolar appearance of the object on a subarcsecond scale. If the envelope's outflow velocity of 15 km s^-1 applies to the material making up the dense core, then just 15 years ago the star was losing mass at a rate of yr^-1. Dust exists in the envelope of IRC + 10216 everywhere from the stellar photosphere up to a distance of 3 pc from the star. The total mass of the envelope lost by the central star is 3 and the dust-to-gas mass ratio is 0.004. The total optical depth toward the star in the visual is 40, in the polar cavities it is 10. The innermost parts of the envelope are optically thick even at 10.7 m due to a strong resonance absorption of silicon carbide grains at that wavelength. In addition to SiC dust, the model contains inhomogeneous grains made of a mixture of SiC and incompletely amorphous carbon with thin [ Mg_0.5Fe_0.5] S mantles. This is the simplest dust mixture required to fit all observations of IRC + 10216 and to correctly interpret the well-known 11.3 m and 27 m emission bands. The dust model found in this study can also be successfully applied to many other carbon stars exhibiting broad emission features in the 10.3-12.6 m and 25-37 m wavelength regions. An important and firm result of our modeling is that the brightest compact peak observed in IRC + 10216 is not the direct light from the underlying central star. In contrast to previous suggestions, the brightest southern component, labeled A in our high-resolution near-infrared images [CITE], is only the radiation emitted and scattered in the optically thinner southern cavity of the bipolar dense shell moving away from the central star. The carbon star is at the position of the fainter component B in our H and K images, which is 0 fraction of arcsecond21 away from A along the symmetry axis. Direct stellar light (component B) is not seen at all in the Hubble Space Telescope 0.8 m and 1.1 m images, being absorbed by the dense dusty material. The even fainter components C and D in the H and K images are probably due to smaller deviations of the dense shell from the spherical shape. IRC + 10216 seems to have entered a phase immediately before moving off the asymptotic giant branch and started developing asymmetries in its envelope.