2010 Annual Science Report
University of Hawaii, Manoa Reporting | SEP 2009 – AUG 2010
Modelling Grain Surface Chemistry in Dense Clouds - Deuteration
Understanding and tracing isotopic abundance patterns provides a key marker for
tracing the history and evolution of planetary systems. In particular deuterium chemistry
is of interest for tracing the origin of water in habitable worlds. The deuterium to hydrogen ratio is set in the big bang, but the relative abundance of deuterium is enhanced in interstellar space in regions of cold molecular clouds. We have been trying to understand the observations of deuterated water in space through a series of chemical models. D/H enhancement in the precursor solar system material gives us starting conditions for the early solar system composition.
Recent observations of deuterated water have revealed that it is consistently less fractionated than other species by factors ranging from 10 to 100. Motivated by this, we have expanded our Monte Carlo accretion limited model to include deuterium chemistry to investigate the possible role of grain surface chemistry in driving this effect. The number of free parameters, whose influence on the grain surface chemistry is investigated, is limited to four parameters. Namely, (1) the hydrogen nuclei density, (2) the atomic D to H ratio in the accreting gas, (3) O3 cascade, and (4) O3 activation barrier. Our results show that water can be much less fractionated than other species due to the effects of the cascade. In essence, the presence of H2 on grain surfaces can convert a large abundance of O3 present on the grain preferentially into H2O. This does depend however on the assumption that the atomic H released by the reaction H2+ OH remains on the surface and available for reaction. Further quantum chemical calculations on the fraction of the reaction energy going into translational motion of H in this reaction as well as molecular dynamic calculations on the fate of such a 'hot’ H atom on a grain surface could elucidate this assumption. An important consequence of this reaction involving H2 is that grain surface chemistry will not form D2O. 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 co-reactants are available for accreting H. The calculated water/methanol fractionation ratio is somewhat higher than that observed for the protostar IRAS 16293 and much higher than observed for the Orion Compact Ridge.