PHOTOEVAPORATION OF DUSTY CLOUDS NEAR ACTIVE GALACTIC NUCLEI

摘要

We investigate the hydrodynamic and line-emitting properties of dusty clouds exposed to an active galactic nucleus (AGN) continuum. Such clouds 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 carried along with the flow and are destroyed by sputtering as the wind heats. In smaller clouds, sputtering regulates the outflow by reducing the radiation force opposing the flow. Cloud evaporation may be fast enough to determine the location of the inner edge of the torus. However, since the evaporation time is much longer than the orbital time, clouds on eccentric orbits can penetrate well inside the inner edge of the torus. Therefore, the ionization structure of the cloud is determined only by the incipient continuum shape. The inner faces of exposed clouds are pressurized primarily by the incident radiation. Radiation pressure on dust grains regulates how gas pressure increases with optical depth. Ionization levels decrease inward, and the bulk of the cloud is molecular and neutral. The effects of dust extinction and high density 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 is a natural place for producing the iron coronal lines often seen in the spectra of AGNs (i.e., [Fe VII] λ6087, [Fe X] λ6375, [Fe XI] λ7892, and [Fe XIV] λ5303).