9.2. Other Models
Below is a short account of models not requiring external confinement of the clouds.
9.2.1 Winds. Several wind models have been proposed for the origin and motion of the clouds. The main problem in most of these is the source of the cloud gas. The possibilities of mass loss from giants and from the surface of accretion disks have been considered. In one such model, a hot supersonic wind shocks against some obstacles (supernovae remnants or OB stellar winds) to form clouds in the cooled postshock gas.
9.2.2 Orbiting clouds. Massive clouds, in parabolic orbits, have been proposed for the line emitting gas. The clouds are confined by a central relativistic wind and lose mass as they fall in. Tidal forces break the clouds into smaller fragments, at about 1 pc. Such clouds are assumed to produce observed line emission at all distances, from 1 kpc to 0.lpc. The line core is produced at large distances, where the velocities are small, and the line wings are produced by fragments, at the standard BLR distance.
9.2.3 Disks. There have been several models relating the line emission to the central, hypothetical accretion disk (chapters 5). Such a configuration provides a large amount of cool gas that needs no external confinement, and can produce line emission provided a suitable energy mechanism exist. Standard thin accretion disk models suggest that the temperature and the flux emitted by the disk, at the typical BLR distance, are too low to be observed. Thus an additional heating source for the gas is required. As explained in chapter 5, this can be achieved if part of the central continuum radiation is back-scattered onto the disk surface at large radii (Fig. 13).
9.2.4 Stars. Stellar atmospheres have been proposed for the BLR clouds. Given the size and density of the broad line clouds, these must be giant size atmospheres. Stellar atmospheres require no confinement, which is the main virtue of this model. The difficulty is the large number of giants (~ 107) needed to explain the observed covering factor in BLRs. Given a normal stellar population, the total cluster mass is enormous and can dominate the dynamics near the center. It has been suggested that radiation pressure driven winds, or accelerated stellar evolution, keep the required number of stars small.
If BLR clouds are indeed stellar atmospheres, the density and pressure profiles may be very different from the ones assumed so far. The implications to the line intensity, variability and profiles are still to be investigated.