RADIATIVE ACCELERATION IN OUTFLOWS FROM BROAD ABSORPTION-LINE QUASI-STELLAR OBJECTS .2. WIND MODELS

We investigate the dynamics of radiatively driven broad absorption-lin e (BAL) outflows in quasi-stellar objects (QSOs) by developing radial and time-independent numerical models. Two limits are explored. The fi rst assumes that the absorbing matter is not forced to comove with the substrate, which provides pressure confinement. This assumption allow s us to explore in detail a case in which the acceleration is entirely due to radiation pressure. Using the parameters inferred from observa tions, we find that under these conditions radiative acceleration (mai nly due to resonance line scattering) can readily accelerate the flow to the observed velocities. An important feature of the noncoupled flo w is that the line profiles tend to stay relatively flat throughout th e velocity interval covered by the line. We discuss how relaxing the a ssumptions of radial symmetry and time independence may help to explai n the structures observed in BALs. In the second class of models, the absorbing flow is assumed to be completely coupled to the substrate in which it is embedded. Aside from being more plausible physically, the se models produce line profiles that trail off at higher velocities, a behavior observed in some BALs. We show that, even if the substrate i s massless, we have to assume a starting radius very close to the infe rred radius of the broad emission-line region (approximately 0.1 pc) i n order to obtain a significant contribution from radiative accelerati on, given a typical AGN spectrum. The reason is that the energy input needed to pressurize the substrate, allowing the flow to become supers onic and to retain a reasonable ionization equilibrium, at the same ti me contributes appreciably to the acceleration. A way to relax the sma ll starting radius constraint is to use a softer ionizing spectrum.