Evolution of Interstellar Ices |
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Authors: | Allamandola Louis J Bernstein Max P Sandford Scott A Walker Robert L |
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Institution: | Astrochemistry Laboratory, NASA Ames Research Center, Mountain View, CA 94035-1000, USA. lallamandola@mail.arc.nasa.gov |
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Abstract: | Infrared observations, combined with realistic laboratory simulations, have revolutionized our understanding of interstellar
ice and dust, the building blocks of comets. Ices in molecular clouds are dominated by the very simple molecules H2O, CH3OH, NH3, CO, CO2, and probably H2CO and H2. More complex species including nitriles, ketones, and esters are also present, but at lower concentrations. The evidence
for these, as well as the abundant, carbon-rich, interstellar, polycyclic aromatic hydrocarbons (PAHs) is reviewed. Other
possible contributors to the interstellar/pre-cometary ice composition include accretion of gas-phase molecules and in situ photochemical processing. By virtue of their low abundance, accretion of simple gas-phase species is shown to be the least
important of the processes considered in determining ice composition. On the other hand, photochemical processing does play
an important role in driving dust evolution and the composition of minor species. Ultraviolet photolysis of realistic laboratory
analogs readily produces H2, H2CO, CO2, CO, CH4, HCO, and the moderately complex organic molecules: CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), R-CN (nitriles), and hexamethylenetetramine (HMT, C6H12N4), as well as more complex species including amides, ketones, and polyoxymethylenes (POMs). Inclusion of PAHs in the ices
produces many species similar to those found in meteorites including aromatic alcohols, quinones and ethers. Photon assisted
PAH-ice deuterium exchange also occurs. All of these species are readily formed and are therefore likely cometary constituents.
This revised version was published online in June 2006 with corrections to the Cover Date. |
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