Astronomy and Astrophysics – Astrophysics
Scientific paper
Oct 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...290.1009k&link_type=abstract
Astronomy and Astrophysics 290, 1009-1018 (1994)
Astronomy and Astrophysics
Astrophysics
72
Molecular Processes, Solar System: Formation, Stars: Circumstellar Matter, Interstellar Medium: Clouds
Scientific paper
Conditions for formation and preservation of amorphous ice formed through condensation of water vapor on a substrate is investigated both theoretically and experimentally. The kinetic consideration of deposition of vapor leads to the condition for formation of amorphous ice on a cold substrate: the flux of water vapor onto the substrate should be larger than a critical flux D_s_/a^4^=F_c_, where D_s_ is the surface diffusion coefficient of water molecules on the substrate and a the lattice constant of the substrate. The validity of the derived condition has been confirmed by measuring the critical flux of water vapor onto a substrate of polycrystalline cubic ice as a function of temperature. From the measured F_c_, a surface diffusion coefficient of H_2_O molecules on polycrystalline cubic ice has been determined. With regard to the preservation condition, we derive a theoretical expression of the time scale t_c_ for crystallization of amorphous ice, which has the same form at low temperatures as the empirical formula used so far. A self-diffusion coefficient of amorphous ice is estimated from a comparison between the theoretical and empirical formulas of t_c_. The results are applied to condensation of ice in molecular clouds, circumstellar envelopes of late-type stars, and the primordial solar nebula. Discussion is given on the crystallinity of ices in these sites.
Greenberg Mayo J.
Kouchi Akira
Kozasa Takashi
Kuroda Takaji
Yamamoto Tadahiro
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