Computer Science
Scientific paper
Aug 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt.........8h&link_type=abstract
Thesis (PhD). RENSSELAER POLYTECHNIC INSTITUTE, Source DAI-B 65/02, p. 788, Aug 2004, 113 pages.
Computer Science
1
Scientific paper
The water ice mantles on interstellar grains trap volatile molecules, such as CO and CH3OH, with an efficiency that depends on the amorphous or crystalline structure of the ice. The ice structure therefore affects the composition of comets formed from the icy grains. We present a detailed study of the processing of mantled grains by shock waves in protoplanetary disks. The grains suffer a sudden increase in temperature that can evaporate the mantles. This is followed by an extended cooling time during which the mantles recondense on timescales comparable to the crystallization timescale for hydrodynamic parameters consistent with the Jupiter-Saturn region of the solar nebula. We evaluate different scenarios for re-deposition of the mantle, and the possibility of re-trapping the co- adsorbing volatiles. The crystallization of ice and the exclusion of volatiles from the matrix may explain the volatile-depleted composition observed recently in Comet C/1999 S4 (LINEAR), an Oort-cloud comet originating from the Jupiter-Saturn region (Mumma et al. 2001). We demonstrate that the bulk ice desorbs for shock speeds greater than a critical value for a given preshock gas density. Crystallization of water ice is most efficient for models that completely remove and re-accrete the mantle. Weakly polar or apolar molecules such as CO will be retained, at least partially, for mantles that do not sublimate, but will be completely lost if the bulk H2O ice is removed in the shock. Strongly polar molecules such as CH3OH will participate in the hydrogen bonding network of the water ice, and will be retained for all shock models considered. We associate hydrodynamic parameters with radial positions in protoplanetary disks by means of a viscous accretion disk model (Aikawa et al. 1998). Pickett et al. (2003) show that shocks due to gravitational instabilities propagate at oblique incidence to the rotation of the disk, thereby causing the shock speeds to be much lower than the Keplerian speed. For a speed of 2 km s-1, crystallization and volatile removal is efficient within the Jupiter-Saturn region of the nebula, but the mantles will remain unprocessed for the conditions near Uranus and beyond.
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