| Research Abstract |
In order to explore the time scale and controlling factors of phases appear during condensation in various stellar environments, experimental approach to get dusts in laboratory was employed. Condensation includes two elementary processes, nucleation and crystal growth, in which condensation is hard to control on the Earth due to the gravitational force. The most abundant elements that form solid materials in the universe are Si, Mg and Fe. Calculation of chemical equilibrium and astrophysical observation around young and evolved stars and interstellar space reveal that forserite, enstatite and metallic iron are the dominant phases. Therefore, experiments of condensation of forsterite, ensatite, and metallic iron dust on substrates without nucleation are planned at low pressure conditions.
At first, development of an experimental equipment was attempted, and an vacuum chamber made of stainless steel with tungsten mesh heated and W and Mo thermal shields were prepared to obtain large amount of hot gas at the condensation spot. The results, however, were found to be unsuitable for condensation of silicate dusts, because the tungsten prevents condensation of oxygen atoms or molecules into silicates. Then, condensation of metallic iron was intended by using iron oxide (Fe2O3) as an evaporation source, and metallic Mo and forsterite for substrates. The source was heated at 1350℃ for 1 to 120 hours, and the reside and condensates were observed with a scanning electron microscopy (SEM) and analyzed with electron back scattered diffraction (EBSD) technique. Condensates were observed on both Mo and forsterite, of which size and amount increase with time, and are determined to be metallic iron. They show cubic shape and the size is larger on forsterite than Mo. The results show that the temperature of substance at a radiation field from a star plays key role on dust growth due to difference in actual temperature.
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