講演者: Ingrid Mann (Muenster University, Germany)   
  題名: Si Nanoparticles in the Solar System? 
  日時: ７月２５日（月）15時から16時すぎ 
  場所: 自然３号館6階６０９ゼミ室





　The observation of dust in the solar system is typically limited to sizes of dust on scales comparable to that of the scattered light and namely Zodiacal light and F-corona brightness describe the particles in the 1 to 100 micrometer sizes range. Dust impact detection from spacecraft ranges to sizes as small as 1/10 of micrometers and below, but so far no clear information is obtained as to whether a lower limit of dust size exists. On the other hand, laboratory studies indicate that nanometer-sized particles, due to quantum confinement effects can show very efficient photoluminescence and therefore, in spite of their small size, may produce observable brightness signals if they exist in space. This fact provided the basis for the discussion of nano-dust causing brightness signals in the interstellar medium. It also inspired some authors to suggest the existence of Silicon nano-particles in the vicinity of the sun in order to explain some features in the coronal brightness observed during solar eclipse and follow up observations have been made by other groups.





　This seminar approaches the question whether nano-particles exist near the sun starting from our knowledge of the solar system dust cloud. We study the sublimation sequence of silicate dust in the inner solar system and show that the resulting material to survive at small distances from the sun is metal oxides while silicon sublimates at moderate temperatures already. That agrees with the understanding that dust sublimation happens gradually and not at a well-defined distance from the sun. From consideration of the laboratory studies that show the generation of Si nanocrystals we conclude that it is not possible to generate Si nanocrystals near the sun in a similar way nor do most other conditions in space resemble the conditions of these laboratory studies. We conclude that there is no observational evidence for the existence of pure Silicon particles near the sun.





　Aside from these considerations of the material sublimation we discuss how nano-particles (possibly of other materials) can form in the interplanetary medium from collisional fragmentation of dust and meteoroids. We further discuss their lifetimes and also the question what forces influence their dynamics and whether they can produce local enhancements of dust density that can be observed. A limitation for observing nano-particles in the inner solar system may also lie in the temperature dependence of their photoluminescence.