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2008 Annual Science Report

University of Hawaii, Manoa Reporting  |  JUL 2007 – JUN 2008

Molecular Deuteration on Grain Surfaces

Project Summary

n recent years, deuterium fractionation has been instrumental in deciphering the main chemical routes in molecular clouds. This fractionation reflects the small zero-point energy difference (a few hundred Kelvin) between deuterated species and fully hydrogenated species. At low temperatures, this can enhance the abundance of deuterium-bearing species by many orders of magnitude. In fact, recent observations of deuterated water have revealed that it is consistently less fractionated than other species by factors ranging from 10 to 100. Paralleling this, large deuterium fractionation effects are seen, most notably for deuterated forms of formaldehyde, methanol and ammonia versus their fully hydrogenated forms. Motivated by this, we have expanded our Monte Carlo accretion model to include deuterium chemistry in order to explore the role of grain surface chemistry in selectively deuterating ammonia and methanol as opposed to water.

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Project Progress

A key aspect of the grain surface chemistry’s role in the selective fractionation of methanol over water centers around the H2 or HD on the grain surface. This adsorbed H2/HD can cause an ozone cascade that preferentially channels a significant fraction of atomic H into reactions with ozone (O3) resulting in water. Paralleling this, atomic deuterium is selectively channeled into reactions with co-reactants (predominantly formaldehyde, methanol and ammonia) that have lower activation barriers as a result of the zero-point energy difference between atomic hydrogen and deuterium. An important consequence of this reaction scheme is that the formation of deuterated water (both HDO and D2O) is not favored. Significant progress on this project has been made in comparing our model results with recent observations of deuterated isotopologues of ammonia and water toward the embedded low mass protostars IRAS 16293-2422.

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Our grain surface model can accurately account for the enhanced deuteration of ammonia while also reproducing the low fractionation of water. This good agreement demonstrates that grain surface reactions are instrumental in causing significant fractionation for a selective set of molecules.