Chemical reactions in protoplanetary accretion disks III. The role of ionisation processes

This paper considers the chemistry in a protoplanetary accretion disk during the viscous stage. The model calculation extends our previous work by including in the chemical reaction network the ionisation processes due to cosmic rays and especially the ionisation due to the extinct radio-nuclides 26Al and 60Fe which are known to have been present at the time of formation of our own solar system. We discuss the details of the ionisation by these nuclei and calculate the corresponding ionisation rates. Additionally our model calculation considers the ionisation due to the local radiation field within the disk and it considers the dust destruction processes. We solve the time dependent rate equations for the chemistry simultaneously with equations for a semi-analytic model for the disk structure. The calculation shows that important chemical processes with respect to the chemistry of nitrogen and sulfur are initiated by the ionisation processes and the subsequent ion-molecule reactions which results in substantial amounts of molecular species which would not be formed in a completely neutral chemistry. Especially we find that significant amounts of NH3 are formed due to ion-molecular reactions in the region where the terrestrial planets are located and that substantial amounts of CS are present in the outer parts of the disk. The C, O, and Si chemistry are found to be not substantially modified by the addition of ionisation processes to the reaction network and our previous results (Finocchi et al. 1997) for the chemistry of these elements are still valid.