Publications
of the
MPIfR
Optical & Infrared
Interferometry Group
A. Men'shchikov, Y. Balega, T. Blöcker,
R. Osterbart, and G. Weigelt:
Structure and physical properties of the
rapidly evolving dusty envelope of IRC+10216 reconstructed by detailed
two-dimensional radiative transfer modeling
Astronomy & Astrophysics 368, 497-526
(2001)
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, 100µm,
and 1.3mm, a 10.5µm 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 130pc, the model of IRC+10216 implies that
the object changes its luminosity between 13000 and 5200Lsun, its
effective temperature between 2800 and 2500K, and its radius between
500
and 390Rsun. 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 15km/s applies to the
material making up the dense core, then just ~15 years ago the
star was losing mass at a rate of
9x10^{-5} Msun/yr.
Dust exists in the envelope of IRC+10216 everywhere from the stellar
photosphere up to a distance of 3pc from the star. The total mass of
the envelope lost by the central star is 3Msun 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
10.7µm 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 MgFeS mantles. This is the simplest dust
mixture required to fit all observations ofIRC+10216 and to correctly
interpret the well-known 11.3µm and 27µm 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
10.3-12.6µm and 25-37µm wavelength regions.
An important and firm result of our modeling is that the brightest
compact peak observed in IRC+10216 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 seen at all in the Hubble Space
Telescope 0.8µm and 1.1µm 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.
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