Carbon grains in the envelope of IRC +10216

Abstract

IRC +10216 is a well-observed carbon star with copious mass loss, surrounded by an expanding envelope of dust and molecular gas. The authors have studied the dust component using radiative transfer calculations by the half-range moment method. The models are sensitive to the choice of grain properties, basically size and composition. Two types of carbon material were considered: "amorphous carbon" and graphite. Small particles of amorphous carbon (radius ≈ 0.05 - 0.1 μm) seem most suitable. The authors conclude that graphite is not a major component of carbon-rich stardust. SiC is also not a major constituent material. These models provide information on the spectrum, the surface brightness, the polarization, and the relative importance of direct starlight, scattered light, and thermal emission. The authors assess the extent to which observations show that a spherically symmetric density distribution is an oversimplification.