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

AGN SEDs generally show a turnover at λ ∼ 1000 Å, implying a maximal accretion disc (AD) temperature of T_max ∼ 50 000 K. Massive O stars display a similar T_max, associated with a sharp rise in a line-driven mass-loss M_wind with increasing surface temperature. AGN AD are also characterized by similar surface gravity to massive O stars. The M_wind of O stars reaches ∼10^-5 M_⊙ yr^-1. Since the surface area of AGN AD can be 10⁶ larger, the implied M_wind in AGN AD can reach the accretion rate M. A rise to M_wind ~ M towards the AD centre may therefore set a similar cap of T_max ∼ 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_wind becomes comparable to M typically at a few tens of GM/c². Thus, the standard thin AD solution is effectively truncated well outside the innermost stable orbit. The calculated AD SED shows the observed turnover at λ ∼ 1000 Å, 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, 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.