Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan.
Division of Theoretical Astronomy, National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan.
Sci Adv. 2017 Jan 18;3(1):e1601992. doi: 10.1126/sciadv.1601992. eCollection 2017 Jan.
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.
宇宙中主要元素的丰富形态已经通过大量的天文观测和陨石分析确定。铁（Fe）是一个例外，因为只在星际介质中检测到气态 Fe 的消耗，这表明 Fe 被凝聚成固体，可能是天文学家看不见的金属。为了确定 Fe 的主要形态，我们通过微重力实验复制了演化恒星气态喷出物中 Fe 颗粒的形成。我们发现 Fe 颗粒形成的附着概率极低；每百万次碰撞只有少数几个原子会附着，因此即使在 Ia 型超新星富含 Fe 的喷出物中，金属 Fe 颗粒的均匀成核也是非常低效的。这意味着大部分 Fe 被锁定在 Fe 化合物的颗粒中，或者作为杂质附着在星际介质中的其他颗粒上。
