Gas clouds from tidally disrupted stars in active galactic nuclei

Abstract

view Abstract Citations (35) References (111) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gas Clouds from Tidally Disrupted Stars in Active Galactic Nuclei Roos, Nico Abstract A direct implication of the presence of massive black holes in galactic nuclei is the occasional tidal disruption of a star in the vicinity of the hole. The tidal disruption rate will be enhanced as a result of perturbations of the star cluster around the hole (for instance during the final stage of a merger with a smaller galaxy) leading to prolonged periods of activity. Recent investigations of the tidal break- up of solar-type stars near massive black holes have shown that debris gas will be distributed throughout the galactic nucleus. About half of the stellar debris mass is strongly bound to the hole and will quickly be swallowed while the rest is expelled with typical velocities of order 5000M^1/6^_6_ km s^-1^, where M_6_ = M_hole_/10^6^ M_sun_. In this paper 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~10^4^K) surrounded by hot gas at the Compton temperature (T_c_~10^7-8^K) when the ionization parameter {XI} (defined as the ratio of the ionizing radiation pressure to the gas pressure) reaches the critical value {XI}_c_ ~ 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 ~10^16-17^ cm in Seyferts and ~10^17-18^ cm in quasars. The broad line region (BLR) 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 ~10^9-11 cm^-3^ and ~10^24-27^ cm^-2^, respectively ({XI}~10). Bound debris orbiting at ~10^15-16^M^2/3^_6_ cm with characteristic densities >= 10^11^ cm^-3^ may contribute to the inner part of the broad line region yielding symmetric lines, whereas a relatively small number (~10-100) of large outmoving remnant clouds produces a more asymmetric and bumpy line profile. There are two potential problems with this new model for the BLR: First, a high tidal disruption rate of order >= 0.1 M_sun_ yr^-1^ seems required to replenish the gas in the BLR in Seyferts as well as in QSOs. Second, in some AGN the central continuum source may be obscured by optically thick remnant clouds along our line of site. A very attractive property of the tidal disruption model is that it offers a natural interpretation not only for the broad emission lines but for a number of other features in AGN spectra as well. Tidal disruption will provide cold, dense gas close to the central source which can reprocess the radiation from the central source and produce a thermal bump in the optical/UV as well as several characteristic X-ray features. Optically thin remnants moving along our line of site outside the BLR at about 10^18-19^ cm may produce broad absorption lines similar to those observed in broad absorption line QSOs. The radially outmoving remnants are decelerated and compressed by the interstellar medium at radii >= 10^19^ cm, where they will emit narrow forbidden lines. Publication: The Astrophysical Journal Pub Date: January 1992 DOI: 10.1086/170919 Bibcode: 1992ApJ...385..108R Keywords: Active Galactic Nuclei; Black Holes (Astronomy); Gravitational Collapse; High Temperature Gases; Interstellar Gas; Star Clusters; Gas Ionization; Gas Pressure; Perturbation Theory; Photoionization; Quasars; Seyfert Galaxies; Spectral Line Width; Tides; Astrophysics; BLACK HOLE PHYSICS; GALAXIES: KINEMATICS AND DYNAMICS; GALAXIES: NUCLEI; GALAXIES: SEYFERT; GALAXIES: QUASARS: GENERAL full text sources ADS | data products SIMBAD (3) MAST (1)