Spectral and spatial distribution of radiation from ETA Carine. I. A spherical dust shell model approach

An extended spherical circumstellar dust shell model in which the radiative transfer problem is treated accurately is developed for eta Car. It is found that both the spectral energy distribution, including the 10 μm silicate feature, and the spatial distribution of radiation can be well represented by the model assuming that the shell consists of a mixture of grains with realistic opacity laws. In particular it is concluded that the satisfactory representation of the 10 μm feature requires, in addition to silicates, grains which possess an emissivity maximum in the 12-14 μm region. The possibility that these grains consist of corundum, and iron grains contribute to the extinction optical depth at 1 μm in the ratio of 0.09:0.09:0.82. On the basis of the predicted shape of the 10 μm feature as would be measured with various apertures it is concluded that there is some evidence for there being an enrichment of corundum grains, relative to the silicate material, in the outer regions. Core-region limb brightening is predicted in the infrared if the shell around eta Car is spherically symmetric, but resolutions beyond the limit of existing telescopes using conventional techniques may be required to reveal it