Broad lines from tidally disrupted stars

Abstract

Recent investigations of the tidal break‐up of solar‐type stars near massive black holes have shown that about half of the stellar debris mass is strongly bound to the hole and will quickly be swallowed while the rest flies away in large, radially elongated clouds with typical velocities of order 5000 M61/6 km s−1, where M6=Mhole/106M⊙. Tidal disruptions of stars near a central massive black hole may therefore provide fuel for AGN and also be a source of cold, dense gas which will emit broad permitted lines when photoionized by the central continuum source (Roos, 1992). The structure and properties of freely expanding (unbound) remnant clouds irradiated by the central continuum source of an AGN are discussed. It is argued that an expanding remnant cloud fragments into cloud clumps of gas (T∼104 K) surrounded by hot gas at the Compton temperature (TC∼107–8 K) when the ionization parameter Ξ (defined as the ratio of the ionizing radiation pressure to the gas pressure) reaches the critical value Ξc∼10. Subsequently the fraction of hot gas slowly increases while the ionization parameter of both components remains equal to Ξc. Photoionization of cold gas in the outmoving and radially elongated remnant clouds produces a broad line region at a distance of ∼1016–17 cm in Seyferts and ∼1017–18 cm in quasars. The broad line region has an outer boundary where the remnant clouds become optically thin to the ionizing radiation from the central source. The densities and column densities in the BLR are in the range ∼109–11 cm−3 and ∼1024–27 cm−2 respectively (Ξ∼10). Bound debris orbiting at ∼1015–16 M62/3 cm may contribute to the inner part of the broad line region. It will yield symmetric lines, whereas a relatively small number (∼10–100) of large outmoving remnant clouds produces a more asymmetric and bumpy line profile. We expect that in some AGN the central continuum source will be obscured by a large remnant cloud on our line of sight.