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Paper: Laboratory Condensation and Reaction of Silicate Dust
Volume: 414, Cosmic Dust—Near and Far
Page: 403
Authors: Nagahara, H.; Ogawa, R.; Ozawa, K.; Tamada, S.; Tachibana, S.; Chiba, H.
Abstract: Condensation and solid-gas reaction experiments in the Mg-Si-O system are carried out to better understand the origin and evolution of silicate dust in protoplanetary discs and outflow of evolved stars, the diversity of which has been revealed by infrared observation. The importance of gas kinematics is stressed: strongly oriented gas flow causes condensation of extremely disequilibrium phases because of the lack of a chance for gas atoms or molecules to interact and to attain equilibrium, whereas, random motion of gas atoms and molecules result in condensation of near equilibrium phases. Contrary to our expectation, Si easily condenses as SiO and Mg hardly condenses in a disequilibrium condition, where Mg is incorporated rather into amorphous silicate. Enstatite formation through reaction between previously condensed crystalline forsterite and gaseous SiO is also controlled by gas kinematics, where highly oriented gas flow prevents the reaction. On the other hand, enstatite is easily formed by reaction with forsterite and SiO gas if gas motion is random. Finally, we have successfully obtained the critical value for condensation (supersaturation ratio) at pressures corresponding to stellar winds of evolved stars.
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