16:45 〜 17:00
[MIS12-11] 重力下および微小重力下におけるチタンおよびカーボンの核生成実験
キーワード：核生成、ダスト、微小重力
Primitive meteorites contain presolar grains, which show isotopic anomaly suggesting formation in the gas outflow of evolved stars and are survivor in building block of the Solar System bodies. Since the characteristics of presolar grains reflect the formation environment, we should be able to know more about the mass loss of corresponding evolved star by analyzing the grains. Titanium carbide with a thick graphitic carbon layer is one of well-known presolar grains and has been called “core-mantle grain”. The core-mantle grain has a carbide with several tens of nanometers in diameter at the center and approximately a micrometer thick graphitic mantle layer. The core-mantle structure indicates that the titanium carbide condensed first and the carbon was deposited later on it. Based on the structure and the size, their formation environment, such as carbon-oxygen ratio, mass-loss rate, temperature and so on, was investigated. However, no suitable formation environment of the core-mantle grain has been found.
To understand formation condition of core-mantle grains, we performed replicate experiments in both gravity and microgravity environments and determined two physical quantities of sticking probability and surface tension, which are crucial in a grain formation model using a nucleation theory. Since the physical quantities depend on temperature, grain size and so on, experimental determination in a similar environment, where grans form, is necessary. Especially, the ratio of collision frequency to cooling timescale is similar between grain formation in gas outflow from an evolved star and microgravity experiment. Therefore, we conducted a microgravity experiment using a MASER 14 sounding rocket and compared the results with that of gravity experiments.
Starting material was evaporated in a buffer argon gas and determined the temperature and concentration during gas cooling by in-situ imaging of a double-wavelength, Mach-Zehnder type laser interferometer. Produced particles were observed using transmission electron microscopes. Based on the nucleation conditions including timescale for gas cools and size of the produced particles, sticking probability and surface tension of nucleated particles are determined using the modified classical nucleation theory (MCNT).
Both of determined physical quantities of sticking probabilities (<0.5) and surface tensions (tend to larger than bulk values) of titan, carbon and titanium carbide suggest inefficient formation of presolar grains. We will discuss how presolar grains can be formed and grown based on the results of the further experiments and theoretical modeling.
Acknowledgements
Developments of the experimental system was supported by the Technical Division of Institute of Low Temperature Science, Hokkaido University, and the Advanced Machining Technology Group of JAXA. Onboard data saving was supported by Dr. S. Takeuchi of ISAS/JAXA. This work was supported by FY2016 ISAS Small-Scale Science Projects of JAXA and Grant-in-Aids for Scientific Research (S) from KAKENHI (15H05731 and 20H05657).
To understand formation condition of core-mantle grains, we performed replicate experiments in both gravity and microgravity environments and determined two physical quantities of sticking probability and surface tension, which are crucial in a grain formation model using a nucleation theory. Since the physical quantities depend on temperature, grain size and so on, experimental determination in a similar environment, where grans form, is necessary. Especially, the ratio of collision frequency to cooling timescale is similar between grain formation in gas outflow from an evolved star and microgravity experiment. Therefore, we conducted a microgravity experiment using a MASER 14 sounding rocket and compared the results with that of gravity experiments.
Starting material was evaporated in a buffer argon gas and determined the temperature and concentration during gas cooling by in-situ imaging of a double-wavelength, Mach-Zehnder type laser interferometer. Produced particles were observed using transmission electron microscopes. Based on the nucleation conditions including timescale for gas cools and size of the produced particles, sticking probability and surface tension of nucleated particles are determined using the modified classical nucleation theory (MCNT).
Both of determined physical quantities of sticking probabilities (<0.5) and surface tensions (tend to larger than bulk values) of titan, carbon and titanium carbide suggest inefficient formation of presolar grains. We will discuss how presolar grains can be formed and grown based on the results of the further experiments and theoretical modeling.
Acknowledgements
Developments of the experimental system was supported by the Technical Division of Institute of Low Temperature Science, Hokkaido University, and the Advanced Machining Technology Group of JAXA. Onboard data saving was supported by Dr. S. Takeuchi of ISAS/JAXA. This work was supported by FY2016 ISAS Small-Scale Science Projects of JAXA and Grant-in-Aids for Scientific Research (S) from KAKENHI (15H05731 and 20H05657).