Line driven winds and the UV turnover in AGN accretion discs
  AGN SEDs generally show a turnover at lambda 1000A, implying a maximal
accretion disc (AD) temperature of T_max~50,000K. Massive O stars display a
similar T_max, associated with a sharp rise in a line driven mass loss
Mdot_wind with increasing surface temperature. AGN AD are also characterized by
similar surface gravity to massive O stars. The Mdot_wind of O stars reaches
~10^-5 Msun/year. Since the surface area of AGN AD can be 10^6 larger, the
implied Mdot_wind in AGN AD can reach the accretion rate Mdot. A rise to
Mdot_wind Mdot towards the AD center may therefore set a similar cap of
T_max~50,000K. 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 Mdot_wind becomes
comparable to Mdot typically at a few 10s 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~1000A, 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 Mdot, 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.
