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Just as the shapes of snowflakes provide us with information on the temperature and humidity of the upper atmosphere, the characteristics of presolar grains in meteorites place limits on their formation environment in a stellar outflow. However, even in the case of well-characterized presolar grains consisting of a titanium carbide core and a graphitic carbon mantle, it is not possible to delimit their formation environment. Here, we have demonstrated the formation of core-mantle grains in gravitational and microgravity environments and have found that core-mantle grains are formed by a nonclassical nucleation pathway involving the three steps: (i) primary nucleation of carbon at a substantially high supersaturation, (ii) heterogeneous condensation of titanium carbide on the carbon, and (iii) fusion of nuclei. We argue that the characteristics of not only core-mantle grains but also other presolar and solar grains might be accurately explained by considering a nonclassical nucleation pathway.

Observations of polar mesospheric clouds have revealed the presence of solid ice particles in the upper mesosphere at high latitudes; however, their formation mechanism remains uncertain. In this study, we investigated the formation process of ice particles through nucleation from small amounts of water vapor at low temperatures. Previous studies that used classical nucleation theory have shown that amorphous solid water particles can nucleate homogeneously at conditions that are present in the mesosphere. However, the rate predictions for water in classical nucleation theory disagree with experimental measurements by several orders of magnitude. We adopted a semi-phenomenological model for the nucleation process, which corrects the evaluation of the molecular cluster formation energy using the second virial coefficient, which agrees with both experiments and molecular dynamics simulations. To calculate the nucleation process, we applied atmospheric conditions for the temperature, pressure, numerical density of dust grains, and cooling rate. The results indicate that homogeneous water nucleation is extremely unlikely to occur in the mesosphere, while heterogeneous nucleation occurs effectively. Dust grains generated by meteor ablation can serve as nuclei for heterogeneous nucleation. We also showed that the ice can form directly in a crystalline state, rather than an amorphous state.

To explain observations of abundant circumstellar dust and high stellar wind velocity, most models simply postulate the efficient nucleation and growth of silicate dust particles. Here, we report measurement of the SiO-(SiOx)(n) grain sticking coefficient in a microgravity sounding rocket experiment, indicating very inefficient (0.005-0.016) grain formation from the vapor. Application of this measurement to radiative-driven winds in oxygen-rich asymptotic giant branch stars indicates that the initial grain condensate population should consist of very tiny dust particles in very large numbers. Aggregation of this dust population will produce low-dimension fractal aggregates that should couple well to the stellar radiation field and efficiently drive stellar mass loss.

To explain observations of abundant circumstellar dust and high stellar wind velocity, most models simply postulate the efficient nucleation and growth of silicate dust particles. Here, we report measurement of the SiO-(SiOx)(n) grain sticking coefficient in a microgravity sounding rocket experiment, indicating very inefficient (0.005-0.016) grain formation from the vapor. Application of this measurement to radiative-driven winds in oxygen-rich asymptotic giant branch stars indicates that the initial grain condensate population should consist of very tiny dust particles in very large numbers. Aggregation of this dust population will produce low-dimension fractal aggregates that should couple well to the stellar radiation field and efficiently drive stellar mass loss.

We propose a novel method for analyzing the crystallization process from supercooled water droplets. The method, which is based on nucleation theory, simultaneously evolves homogeneous ice nucleation and crystal growth in the cooling process and obtains the crystallization temperature and the number of crystal nuclei in the droplets. The model can reproduce not only the crystallization of water but also the vitrification process. The model well replicated the results of previous laboratory experiments, especially, the different responses of the crystallization temperatures of the micrometer- and nanometer-sized particles as a function of cooling rates. For particle sizes ranging from 1 to 1000 m and cooling rates below 10(4) K s(-1), the crystallization temperature was 230-240 K. At cooling rates above 10(4) K s(-1), the crystallization temperature decreased rapidly. On the other hand, the crystallization temperature of 10 nm particles was 200-230 K at cooling rates below 10(4) K s(-1). When describing the interfacial tension by sigma = 29.1 + 0.1(T - 273.15) erg cm(-2) (where T is the water droplet temperature in K), the analyses explained well the previously reported crystallization temperatures of droplets sized from a few nm to 100 m under various cooling conditions. Our model also predicts the critical cooling rate for vitrification of the liquid water droplets. The critical cooling rate of vitrification is predicted to be 10(7)-10(8) K s(-1), consistent with the experimental rates. These analyses are useful not only for comprehensively understanding the ice nucleation process but also for predicting the crystallization processes in various environments such as cirrus clouds, which are difficult to reproduce in experiments.

We propose a novel method for analyzing the crystallization process from supercooled water droplets. The method, which is based on nucleation theory, simultaneously evolves homogeneous ice nucleation and crystal growth in the cooling process and obtains the crystallization temperature and the number of crystal nuclei in the droplets. The model can reproduce not only the crystallization of water but also the vitrification process. The model well replicated the results of previous laboratory experiments, especially, the different responses of the crystallization temperatures of the micrometer- and nanometer-sized particles as a function of cooling rates. For particle sizes ranging from 1 to 1000 m and cooling rates below 10(4) K s(-1), the crystallization temperature was 230-240 K. At cooling rates above 10(4) K s(-1), the crystallization temperature decreased rapidly. On the other hand, the crystallization temperature of 10 nm particles was 200-230 K at cooling rates below 10(4) K s(-1). When describing the interfacial tension by sigma = 29.1 + 0.1(T - 273.15) erg cm(-2) (where T is the water droplet temperature in K), the analyses explained well the previously reported crystallization temperatures of droplets sized from a few nm to 100 m under various cooling conditions. Our model also predicts the critical cooling rate for vitrification of the liquid water droplets. The critical cooling rate of vitrification is predicted to be 10(7)-10(8) K s(-1), consistent with the experimental rates. These analyses are useful not only for comprehensively understanding the ice nucleation process but also for predicting the crystallization processes in various environments such as cirrus clouds, which are difficult to reproduce in experiments.

会議情報: 第1回観測ロケットシンポジウム（2018年7月17日-18日. 宇宙航空研究開発機構宇宙科学研究所（JAXA)(ISAS)）, 相模原市, 神奈川県, 著者人数: 16名, Meeting Information: 1st Sounding Rocket Symposium (July 17-18, 2018. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan

会議情報: 第31回宇宙環境利用シンポジウム (2017年1月16日-17日. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS)), 相模原市, 神奈川県, Meeting Information: Space Utilization Research (January 16-17, 2017. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS)), Sagamihara, Kanagawa Japan

The abundant forms in which the major elements in the universe exist have been determined from numerous astronomical observations and meteoritic analyses. Iron (Fe) is an exception, in that only depletion of gaseous Fe has been detected in the interstellar medium, suggesting that Fe is condensed into a solid, possibly the astronomically invisible metal. To determine the primary form of Fe, we replicated the formation of Fe grains in gaseous ejecta of evolved stars by means of microgravity experiments. We found that the sticking probability for the formation of Fe grains is extremely small; only a few atoms will stick per hundred thousand collisions so that homogeneous nucleation of metallic Fe grains is highly ineffective, even in the Fe-rich ejecta of type Ia supernovae. This implies that most Fe is locked up as grains of Fe compounds or as impurities accreted onto other grains in the interstellar medium.