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

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 accoun t as many observational constraints as possible. The model reproduces very well the entire beam-matched spectral energy distribution of IRC +10 216 fr om optical to centimeter wavelengths (at several phases of stellar luminosi ty), observed intensity profiles of the object at 1.25, 2.2, 10.5, 50, 100 mum, and 1.3 mm, a 10.5 mum lunar occultation intensity profile, our high-r esolution J,H, K, and H-K bispectrum speckle-interferometry images, and vis ibilities in J, H, K, L, M, and N bands. For the adopted distance of 130 pc , the model of IRC + 10216 implies that the object changes its luminosity b etween 13000 and 5200L(circle dot), its effective temperature between 2800 and 2500 K, and its radius between 500 and 390 R-circle dot. There is a den se non-spherical dust shell around the star, with outflow cavities at posit ion angle PA approximate to 20 degrees. The southern cavity with a full ope ning angle of 36 degrees is tilted toward us by 400 from the plane of sky, causing the observed bipolar appearance of the object on a subarcsecond sca le. If the envelope's outflow velocity of 15 km s(-1) applies to the materi al making up the dense core, then just similar to 15 years ago the star was losing mass at a rate of 9 10(-5) M(circle dot)yr(-1). Dust exists in the envelope of IRC+10216 everywhere from the stellar photosphere up to a dista nce of 3 pc from the star. The total mass of the envelope lost by the centr al star is 3 Mo and the dust to-gas mass ratio is 0.004. The total optical depth tau (V) 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 mum due to a strong resonance absorption of silicon carbide grains at tha t wavelength. In addition to SiC dust, the model contains inhomogeneous gra ins made of a mixture of SiC and incompletely amorphous carbon with thin [M g(0.5)aFe(0.5)]S mantles. This is the simplest dust mixture required to fit all observations of IRC +10 216 and to correctly interpret tile well-known 11.3 mum and 27 mum emission bands. The dust model found in this study can also be successfully applied to many other carbon stars exhibiting broad e mission features in the 10.3-12.6 mum and 25-37 mum wavelength regions. An important and firm result of our modeling is that tile brightest compact pe ak observed in IRC +10216 is not the direct light from the underlying centr al star. In contrast to previous suggestions, the brightest southern compon ent, labeled A in our high-resolution near-infrared images (Weigelt et al. 1998a,b; Osterbart et al. 2000), is only the radiation emitted and scattere d in the optically thinner southern cavity of the bipolar dense shell movin g away from the central star. The carbon star is at the position of the fai nter component B in our H and K images, which is 0." 21 away from A along t he symmetry axis. Direct stellar light (component B) is not seen at ail in the Hubble Space Telescope 0.8 mum and 1.1 mum images, being absorbed by th e 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 s pherical shape. IRC +10216 seems to have entered a phase immediately before moving off the asymptotic giant branch and started developing asymmetries in its envelope.