• Condensation processes in astrophysical environments: The composition and structure of cometary grains

      Nuth, Joseph A.; Rietmeijer, Frans J. M.; Hill, Hugh G. M. (The Meteoritical Society, 2002-01-01)
      We review the results of our recent experimental studies of astrophysical dust analogs. We discuss the condensation of amorphous silicates from mixed metal vapors, including evidence that such condensates form with metastable eutectic compositions. We consider the spectral evolution of amorphous magnesium silicate condensates as a function of time and temperature. Magnesium silicate smokes anneal readily at temperatures of about 1000-1100 K. In contrast we find that iron silicates require much higher temperatures (~1300 K) to bring about similar changes on the same timescale (days to months). We first apply these results to ISO observations of crystalline magnesium silicate grains around high-mass-outflow AGB stars in order to demonstrate their general utility in a rather simple environment. Finally, we apply these experimental results to infrared observations of comets and protostars in order to derive some interesting conclusions regarding large-scale nebular dynamics, the natural production of organic molecules in protostellar nebulae and the use of crystalline magnesium silicates as a relative indicator of a comet's formation age.
    • Formation of TiC core-graphitic mantle grains from CO gas

      Kimura, Yuki; Nuth, Joseph A.; Ferguson, Frank T. (The Meteoritical Society, 2006-01-01)
      We demonstrate a new formation route for TiC core-graphitic mantle spherules that does not require carbon-atom addition and the very long time scales associated with such growth (Bernatowicz et al. 1996). Carbonaceous materials can be formed from C2H2 and its derivatives, as well as from CO gas. In this paper, we will demonstrate that large-cage-structure carbon particles can be produced from CO gas by the Boudouard reaction. Since the sublimation temperature for such fullerenes is low, the large cages can be deposited onto previously nucleated TiC and produce TiC core-graphitic mantle spherules. New constraints for the formation conditions and the time scale for the formation of TiC core-graphitic mantle spherules are suggested by the results of this study. In particular, TiC core-graphitic mantle grains that are found in primitive meteorites that have never experienced hydration could be mantled by fullerenes or carbon nanotubes rather than by graphite. In situ observations of these grains in primitive anhydrous meteoritic matrix could confirm or refute this prediction and would demonstrate that the graphitic mantle on such grains is a metamorphic feature due to interaction of the presolar fullerenes with water within the meteorite matrix.