Structure and stability of quasar clouds

Abstract

The structure of broad emission-line clouds is discussed, including radiative forces and dynamical pressure. A simplified model is developed for the radial variation of pressure and density and the associated radial equation of motion of the cloud. Assumptions are made which enhance the likelihood of coherent radial acceleration in the quasar atmosphere, but cloud coherence is problematical. Absorption of momentum from X-rays results in internal pressure maxima which generate lateral flows in the clouds. The rim regions around the transverse periphery of radially flattened clouds may be pushed away from the clouds by differential radiative forces in t(r) about 10 times the radial sound crossing time. Such a derimming process in optically thin clouds could elongate them during their radial acceleration, but without disturbing the resulting emission-line profile. The derimming process can significantly distort optically thick clouds, affecting their emission-line profiles. Optically thick clouds must then be accelerated by a wind, since radiative forces alone cannot generate the observed cloud velocities in t less than t(r). The combined effect of dynamical and radiative forces can accelerate clouds coherently in t approximately less than t(r), producing broad logarithmic line profiles. Wind acceleration is Rayleigh-Taylor stable if radiation forces are included. Cloudsmore » in nonradial orbits are likely to be disrupted by Keplerian shear, differential radiation forces, collisions, or large drag forces. Clouds probably do not arise from cooling condensations or from virially moving objects if they are created close to the central object. 37 references.« less