2009 Annual Science Report
University of Hawaii, Manoa Reporting | JUL 2008 – AUG 2009
Modelling Grain Surface Chemistry in Dense Clouds
Dust grains in interstellar molecular clouds accrete molecules from the gas,
resulting in the growth of ice mantles that eventually get
transported into the protostellar environment. It is here, that the
warm and dense environments of star forming regions promote a rich
chemistry that creates complex prebiotic compounds and a small
fraction of this material ends up as planets. Quantifying the level of molecular complexity attainable through dust grain surface chemistry requires modeling the key
hydrogenation and oxidation chemical pathways.
Additional reactions were add to and modifications in the ozone cascade sequence were made to our existing in-house Monte Carlo grain surface chemistry code. No changes to our early results were detected. Traditionally, it has been assumed that CO2 ice is formed by atomic O reacting with CO. However, laboratory studies do not support this formation mechanism. An important finding from this study is that CO + OH can reasonably account for the observed CO and CO2 ice abundances. If O + CO is responsible for CO2 then trace species with appreciable abundances, including NH2CHO and H2O2 are formed. The O2/O gas phase ratio is large so that N preferentially reacts with HCO (as opposed to O) and, as a result of the large build-up of O2 on the surface, H + O2 becomes a dominant reaction. On the other hand, if OH + CO forms CO2 then we find HCOOH and NH2CHO but not H2O2 . In this case, there is more atomic O in the gas phase that, once it is accreted, can react with the HCO radical. A concerted effort should be made to search for signatures of these trace molecules which will then better constrain the chemical reactions that occur on grain surfaces.