Gas clouds from tidally disrupted stars in active galactic nuclei

Abstract

The structure and properties of freely expanding (unbound) remnant clouds irradiated by the central continuum source of an AGN are explored. It is argued that the remnant cloud fragments into cold clumps of gas (T is approximately equal to 10,000 K) surrounded by hot gas at the Compton temperature (about 10 exp 7-8 K) when the ionization parameter Xi(c) (defined as the ratio of the ionizing radiation pressure to the gas pressure) reaches the critical value of about 10. Subsequently the fraction of hot gas slowly increases at the expense of the cold component while the ionization parameter of both components remains equal to Xi(c). Photoionization of cold gas in the outmoving and radially elongated remnant clouds produces a broad line region at a distance of about 10 exp 16-17 cm in Seyferts and about 10 exp 17-18 cm in quasars. A tidal disruption model is presented which offers a natural interpretation for both the broad emission lines and a number of other features in AGN spectra.