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

Abstract

Abstract. We present the first detailed, two-dimensional radiative transfer model of the dusty envelope around the carbon star IRC +10 216. 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 +10 216 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 µm, and 1.3 mm, a 10.5 µm lunar occultation intensity profile, our high-resolution J, H, K, and H −K bispectrum speckle-interferometry images, and visibilities in J, H, K, L, M , and N bands. For the adopted distance of 130 pc, the model of IRC +10 216 implies that the object changes its luminosity between 13 000 and 5200 L , its effective temperature between 2800 and 2500 K, and its radius between 500 and 390 R . There is a dense non-spherical dust shell around the star, with outflow cavities at position angle PA ≈ 20• . 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 9 10 −5 M yr −1 . Dust exists in the envelope of IRC +10 216 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 M and the dust-to-gas mass ratio is 0.004. The total optical depth τV 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 µm 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 [Mg0.5Fe0.5]S mantles. This is the simplest dust mixture required to fit all observations of IRC +10 216 and to correctly interpret the well-known 11.3 µm and 27 µm 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 µm and 25-37 µm wavelength regions. An important and firm result of our modeling is that the brightest compact peak observed in IRC +10 216 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 (Weigelt et al. 1998a,b;Osterbart et al. 2000), 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. 21 away from A along the symmetry axis. Direct stellar light (component B) is not se...

Near-infrared speckle interferometry and radiative transfer modelling of the carbon star LP Andromedae

“…As the envelopes are optically thick, they reprocess the optical radiation of the central star into the infrared, where they are very bright. The best-studied object of this type is the carbon star CW Leo, also known as IRC+10 216 (see, e.g., Men'shchikov et al 2001Men'shchikov et al , 2002aWeigelt et al 2002, and references therein), whose many properties are remarkably similar to those of LP And, as it will be shown in this paper.…”

“…Not only are the SEDs of the stars almost identical, but other parameters, such as their pulsation periods, luminosities, effective temperatures, masses, mass-loss rates, outflow velocities, and evolutionary stages (both are carbon stars) are also very similar. Thus, our idea was to use the detailed dust envelope model of CW Leo presented by Men'shchikov et al (2001Men'shchikov et al ( , 2002a as the basis for our numerical study of LP And. We refer to the papers for a more detailed description of the model geometry, dust properties, and modelling approach, as well as for a discussion of all assumptions.…”

“…The total mass of the envelope is M = 3.2 M within its outer radius R 2 = 2.9 pc, almost the same as in CW Leo. The mass is based on the same value of ρ d /ρ = 0.0039 in the outer envelope and the same dust model as in Men'shchikov et al (2001). These assumptions are justified by the fact that the observed infrared emission bands are extremely similar in both LP And and CW Leo.…”

“…As in CW Leo, both the radial distribution and composition of dust in our model of LP And are strongly inhomogeneous. The emission feature at 11.3 µm was best reproduced by Men'shchikov et al (2001) by [SiC, C] particles, which are unorganized aggregates made of incompletely amorphous carbon grains with significant graphitic content and silicon carbide grains. The inhomogeneous material was modelled with volume fractions of 15% for α-SiC (Choyke & Palik 1985) and of 85% for the carbon material cel800 (Jäger et al 1998).…”

“…The broad emission feature between 24 and 37 µm was best reproduced by [SiC, C] The abundances of the dust components and the grain size distribution dn/da ∝ a γ exp (−a/a exp ) in our model depend on the distance from the star. The dust-to-gas mass ratios ρ d /ρ, material densities ρ gr , limiting grain radii a min and a exp , and exponents γ of the size distribution for the three dust components are all the same as in Table 2 of Men'shchikov et al (2001). The only difference is that the transition to larger outer dust grains occurs at r = 80 AU in this model of LP And, instead of 100 AU in CW Leo.…”