THE ROLE OF RADIATIVE ACCELERATION IN OUTFLOWS FROM BROAD ABSORPTION-LINE QSOS .1. COMPARISON WITH O-STAR WINDS

We investigate the role of radiation pressure in accelerating the broa d absorption line (BAL) outflows in QSOs by comparing their properties with those of radiatively driven O star winds. We find that, owing to their lower column densities and higher velocity spreads, BAL outflow s have only a few tens of strong resonance lines that are dynamically important, as compared with 10(3)-10(4) lines in O star winds. We show that the combined radiative force (the ''force multiplier'') declines more rapidly as a function of column density for BAL outflows than fo r O star winds. This is mainly attributed to the absence of lines from excited states in the BAL region. The absorbing gas in BAL outflows m ust have a small filling factor in order for radiative acceleration to be important dynamically. This allows the absorbing material to remai n at a high enough density to maintain the ion species necessary for e fficient radiative acceleration (as well as those responsible for the observed absorption), without the average flow density becoming so lar ge that the absorbing matter cannot be accelerated by an increment lar ger than its own sound speed. The latter condition is necessary if the outflow is to tap a large portion of the incident photon momentum. On ce a small filling factor is assumed, radiative acceleration can be mo re efficient in BAL outflows than in O stars. We show that terminal ve locities of a few times 10(4) km s-1 can be expected, provided that th e absorbing matter does not have to drag with it a much heavier substr ate.