Institute for Computational Astrophysics

Understanding the Carbon Star IRC+10216 and its Dusty Envelope

Friday 7th November, 2003
3 pm, McNally Main (MM) 310, SMU campus

Understanding the Carbon Star IRC+10216 and its Dusty Envelope

Dr. Alexander Men'shchikov,
Institute for Computational Astrophysics and
Department of Astronomy & Physics,
Saint Mary's University

I will present the new near-infrared images and detailed two-dimensional radiative transfer model of the dusty environment of the prototype carbon star IRC+10216. The speckle-interferometry images with resolution as high as 50 mas, obtained over the last 5.4 years, revealed a dynamic evolution of the subarcsecond dusty environment of IRC+10216. The radiative transfer modeling shows that 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 deg from the plane of sky, causing the observed bipolar appearance of the object on a subarcsecond scale. 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 Msun 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 mic 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 of IRC+10216 and to correctly interpret the well-known 11.3 mic and 27 mic emission bands. The brightest compact peak observed in IRC+10216 is not the direct light from the underlying central star, but rather the radiation emitted and scattered in the optically thinner southern cavity of the bipolar dense shell. 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 HST 0.8 mic and 1.1 mic images, being absorbed by the dense dusty material. We are witnessing an episode of a steadily increasing mass loss from the central star, from Mdot ~ 10^-5 Msun/yr to the rate of Mdot ~ 3x10^-4 Msun/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 bright cavity A away from the star.