The Physics and Chemistry of Interstellar Grains. I. Crystal Growth in Space

Abstract

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Physics and Chemistry of Interstellar Grains. I. Crystal Growth in Space Donn, Bertram Abstract Experimental and theoretical developments since about 1945 in (1) nucleation of condensing vapors, (2) mechanism of crystal growth, (3) chemical reactions at low temperatures including surface reactions of free radicals, and (4) radiation chemistry of solids, now permits much more reliable analyses of grains to be carried out than were previously possible. We consider the simplest case of growth of pure crystals without reactions. Molecular impact frequencies, surface lifetimes and diffusion rates are calculated. Growth rates and shapes are estimated according to which of the two crystal growth mechanisms is appropriate, (1) trapping with little mobility, producing nearly spherical particles, or (2) dislocation trapping yielding nonspherical grains. Process 2 requires a significant diffusion distance. At 500K all abundant metals except aluminum, iron, and nickel would diffuse and tend to develop irregular particles. At 20~K process (1) would apply to all metals. Molecules of hydrogen with carbon, oxygen, and nitrogen would probably diffuse. At 5 K only molecules more volatile than CO2 undergo diffusion. ~latelets permit sufficient mass for water, ammonia, methane, or graphite grains to produce interstellar extinction. At 20~K the expected surface diffusion of hydrogen compounds is about 1 mm in 10 yr. Carbon atoms on the hexagonal graphite face appear to be similar. Trapping occurs on crystal edges, and platelets should tend to form. A small number of screw dislocations would lead to highly irregular whisker arrays. Exponential growth rates occur. For platelets with N (H) = 1/cm3, r reaches I0-~ cm in about 3x108 yr or 3x107 yr if N (H) =10. Inclusion of chemical reactions, heterogeneous composition, and radiation chemical effects should make growth less favorable. Also recent extinction measurements in the ultraviolet cannot be explained by uniform grains. Consideration should be given to extinction by Platt-type grains. Publication: The Astronomical Journal Pub Date: 1965 DOI: 10.1086/109645 Bibcode: 1965AJ.....70S.320D full text sources ADS |