THE DEPENDENCE OF H_2CO AND CH_3OH FORMATION ON THE TEMPERATURE AND THICKNESS OF H_2O-CO ICE DURING THE SUCCESSIVE HYDROGENATION OF CO

摘要

The formation of formaldehyde (H_2CO) and methanol (CH_3OH) by the successive addition of hydrogen atoms to CO molecules in H_2O-CO mixed ice was measured at 10, 15, and 20 K. The maximum yield of CH_3OH was obtained at 15 K, with the next highest yield at 10 K. At 20 K, the reactions proceeded very slowly, and the resulting yield of CH_3OH was significantly smaller than at 10 and 15 K and was considered to be due to a fall in the sticking probability of hydrogen at this temperature. The observed temperature dependence indicates that the abundances of H_2CO and CH_3H in a molecular cloud are strongly dependent on the temperature of dust as well as the flux of atomic hydrogen. The diffusion of hydrogen atoms in ice is also discussed based on measurements of the dependence of CO hydrogenation on ice thickness in the range of less than 30 monolayers.