The Infrared Characteristics of Circumstellar Silicate Grains

Abstract

We have made a theoretical study of the infrared emission from circumstellar shells around late-type giants, with the aim of deriving the infrared characteristics of the silicate grains condensing in these shells. A large grid of models has been compared with observations of optically visible Miras, IRC sources and OH/IR stars. From fitting the observed relation between the color temperature and the strength of the 10μm feature, it is concluded that the ratio of the 3.5 to 10μm absorption efficiencies of the dust is about 0.25, a factor of 2 less than a previous determination. Detailed modeling of the 2 to 13μm spectrum of OH26.5+0.6, IRC+10011 and R Cas yielded a similar ratio. These detailed models also show that the shape of the 10μm feature, particularly around 8 and 13μm, varies from source to source. The derived 10μm feature is narrower for larger dust column densities. These observed differences in the intrinsic shape of the 10μm feature are not due to differences in size of the condensing particles. Probably they are related to structural or compositional differences in the condensing silicates.