Abstract
The shape, crystallinity, and orientation of dust grains are the important factors that control the IR spectrum from discs or stellar shells of young and evolved stars. We have been working of condensation experiments on forsterite, the most abundant crystalline silicate observed so far. Single crystal of forsterite was heated to evaporate in an aluminum tube and obtained condensates on a Molybdenum substrate, of which temperatures ranged from 1500C to 1400C. Calcium was found to be present, which is estimated to be contaminated from the aluminum tube, and formed crystalline akermanite (Ca2MgSi2O7) at the highest temperature position, and the Ca abundance decreased quickly with lowering temperature. The condensates at temperatures below 1500C are rounded objects, being mixtures of crystalline forsterite and probable amorphous Si-rich material, of which bulk Mg/Si ratio decreases with lowering temperature. The result is consistent with astronomical observation that crystalline forsterite and amorphous Si-rich material coexist in the circumstellar disc of evolved stars.
