2009 Annual Science Report
University of Hawaii, Manoa Reporting | JUL 2008 – AUG 2009
Deuteration on Grain Surfaces
The gas and dust in the interstellar medium undergoes considerable processing in the passage through a cold molecular cloud to a circumstellar envelope to protoplanetary disk to, ultimately, comets and planetesimals. Over the years, deuterium fractionation has been instrumental in deciphering the chemical routes in molecular clouds. Deuterated molecules have a highly temperature-sensitive chemistry and can provide valuable information on physical conditions in the early solar nebula. At low temperatures, the abundance of certain deuterium-bearing species is enhanced by many orders of magnitude. Understanding the discrepancy between the D/H ratio in comets and Earth’s oceans requires better knowledge of grain surface chemistry involving deuterium.
As noted in our work on modelling grain surface chemistry, the H2O grain surface fractional abundance remains large at the higher densities as a direct result of reactions with O3 (R67: H + O3 ——> OH + O2) and H2 (R135: OH + H2 ——> H2O + H). The free H associated with H2 is released back into the reaction scheme and preferentially selects O3 (from R67) since this energy barrier is the lowest of all possible reactants. This in turn produces O2 and OH that can again react with H2 in R135. This is referred to as the O3 cascade since H is continuously released into the system. On the average, once the cascade is started, some 5 additional O3 molecules will be converted into H2O using H from H2. The cascade is only be broken when the H from is absorbed by a coreactant which does not have an activation barrier, or alternatively by a molecule that has a surface concentration (e.g., CO or H2CO) large enough to be competitive with the low activation barrier of R67. Most of the hydrogen present in the system is coming from H2. Yet, its abundance is not important enough to reduce all O3 and CO species. In these conditions, the O2 abundance builds up compared to the lower densities. Due to the lower activation barrier of O3, H reacts more often with ozone than with CO. As the density increases, the amount of O3 on the grain surface increases and the cascade starts to become more important. As a result, most of the available atomic H reacts with O3 forming H2O while the D atoms remain available for selective incorporation into other molecules. The trapping of D in methanol (and organics molecules) peaks at intermediate densities as this is when more coreactants are available for accreting H. Our results show that water is significantly less fractionated than other species due to the effects of the cascade.