Line-driven winds and the UV turnover in AGN accretion discs

AGN SEDs generally show a turnover at lambda similar to 1000 angstrom, implying a maximal accretion disc (AD) temperature of T-max similar to 50 000 K. Massive O stars display a similar T-max, associated with a sharp rise in a line-driven mass-loss. (M) over dot(wind) with increasing surface temperature. AGN AD are also characterized by similar surface gravity to massive O stars. The. (M) over dot(wind) of O stars reaches similar to 10(-5) M-circle dot yr(-1). Since the surface area of AGN AD can be 10(6) larger, the implied (M) over dot(wind) in AGN AD can reach the accretion rate (M) over dot. A rise to (M) over dot(wind) similar to (M) over dot towards the AD centre may therefore set a similar cap of T-max similar to 50 000 K. To explore this idea, we solve the radial structure of an AD with a mass-loss term, and calculate the implied AD emission using the mass-loss term derived from observations of O stars. We find that (M) over dot(wind) becomes comparable to (M) over dot typically at a few tens of GM/c(2). Thus, the standard thin AD solution is effectively truncated well outside the innermost stable orbit. The calculated AD SED shows the observed turnover at lambda similar to 1000 angstrom, which is weakly dependent on the AGN luminosity and black hole mass. The AD SED is generally independent of the black hole spin, due to the large truncation radius. However, a cold AD (low (M) over dot, high black hole mass) is predicted to be windless, and thus its SED should be sensitive to the black hole spin. The accreted gas may form a hot thick disc with a low radiative efficiency inside the truncation radius, or a strong line-driven outflow, depending on its ionization state.