Ice Formation in Radiated Accretion Disks

Gas to solid phase changes of abundant species in a viscous, irradiated protoplanetary disk are investigated using a new formulation for the freezeout effect. The method is based on a procedure using species-dependent phase diagrams while following the chemical evolution of water and carbon monoxide gas until their partial pressures are sufficient to de-sublimate vapor into ice. It is found that water ice is dominant throughout the nebula while a significant amount of water vapor coexists with the ice in the cooler parts of the inner nebula. Volatile CO molecules de-sublimate only in the colder outer regions of the nebula near the center plane. Computed column densities for CO gas are compared with similar calculations using an adsorption/desorption model by Aikawa and Herbst and are shown to predict similar distributions.