Chemical evolution of the gas disks

Models of irradiated disks predict four chemically distinct zones (see Fig. 4.1). (I) Zone of ices in the cold mid-plane opaque to incoming radiation. Chemistry in this region is dominated by cold gas-phase and grain-surface reactions. Here Infrared Space Observatory (ISO) and Spitzer observations confirmed the existence of ices, various silicates and PAHs (polycyclic aromatic hydrocarbons e.g. van den Ancker et al. 2000 van Dishoeck 2004 Bouwman et al. 2008). (II) Zone of molecules, a warm molecular layer adjacent to the mid-plane, dominated by ultraviolet/X-ray-driven photochemistry (III) the heavily irradiated zone of radicals, a hot dilute disk atmosphere deficient in molecules and (IV) the inner zone, inside of the ice line where terrestrial planets form. 

Mid- and far-infrared, submillimeter, and radio-wavelength observations allow probing the presence and abundance of simple molecules in these zones and provide constraints and boundary conditions for coupled disk evolution and chemical network models. The observed abundances and predictions from the disk models can be directly compared to constraints derived from the early Solar System. 

---