CHEMICAL-REACTIONS IN PROTOPLANETARY ACCRETION DISKS .3. THE ROLE OF IONIZATION PROCESSES

This paper considers the chemistry in a protoplanetary accretion disk during the viscous stage. The model calculation extends our previous w ork by including in the chemical reaction network the ionisation proce sses due to cosmic rays and especially the ionisation due to the extin ct radio-nuclides Al-26 and Fe-60 which are known to have been present at the time of formation of our own solar system. We discuss the deta ils of the ionisation by these nuclei and calculate the corresponding ionisation rates. Additionally our model calculation considers the ion isation due to the local radiation field within the disk and it consid ers the dust destruction processes. We solve the time dependent rate e quations for the chemistry simultaneously with equations for a semi-an alytic model for the disk structure. The calculation shows that import ant chemical processes with respect to the chemistry of nitrogen and s ulfur 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. E specially we find that significant amounts of NH3 are formed due to io n-molecular reactions in the region where the terrestrial planets are located and that substantial amounts of CS are present in the outer pa rts of the disk. The C, O, and Si chemistry are found to be not substa ntially modified by the addition of ionisation processes to the reacti on network and our previous results (Finocchi et al. 1997) for the che mistry of these elements are still valid.