PHOTOEVAPORATION OF DUSTY CLOUDS NEAR ACTIVE GALACTIC NUCLEI

We investigate the hydrodynamic and line-emitting properties of dusty clouds exposed to an active galactic nucleus (AGN) continuum. Such clo uds may be found on the inner edges of the tori commonly implicated in AGN unification schemes. An X-ray-heated wind will be driven off the surface of such a cloud, eventually destroying it. Dust grains are car ried along with the flow and are destroyed by sputtering as the wind h eats. In smaller clouds, sputtering regulates the outflow by reducing the radiation force opposing the flow. Cloud evaporation may be fast e nough to determine the location of the inner edge of the torus. Howeve r, since the evaporation time is much longer than the orbital time, cl ouds on eccentric orbits can penetrate well inside the inner edge of t he torus. Therefore, the ionization structure of the cloud is determin ed only by the incipient continuum shape. The inner faces of exposed c louds are pressurized primarily by the incident radiation. Radiation p ressure on dust grains regulates how gas pressure increases with optic al depth. Ionization levels decrease inward, and the bulk of the cloud is molecular and neutral. The effects of dust extinction and high den sity suppress the hydrogen recombination lines and the forbidden lines from C, N, and O ions below observed levels despite the high covering factor expected for the torus. However, the inner edge of the torus i s a natural place for producing the iron coronal lines often seen in t he spectra of AGNs (i.e., [Fe VIII] lambda 6087, [Fe X] lambda 6375, [ Fe XI] lambda 7892, and [Fe XIV] lambda 5303).