On the transition from efficient to inefficient line-driving in irradiated flows

Observations of ionized AGN outflows have provided compelling evidence that the radiation field transfers both momentum and energy to the plasma. At parsec scale distances in AGN, energy transfer can dominate, in which case the only force needed to launch an outflow is that from gas pressure. Much closer to the black hole, gravity dominates thermal energy due to insufficient heating by the radiation and the gas is in the so-called `cold' regime. Only magnetic or radiation forces can then lead to outflow, but it is unclear at what temperature and ionization state the radiation force weakens, as these properties depend on the spectral energy distribution (SED). In this work, we survey the parameter space of radiation forces due to spectral lines resulting from blackbody SEDs with varying temperatures in the range $\sim 10^4 - 10^6$~K to identify the radiation temperature at which line-driving begins to lose efficiency. We find that the temperature $\lesssim4\times10^5$~K marks the transition to inefficient line driving. We also self-consistently compute the heating and cooling balance to estimate the gas temperature, so that our parameter survey covers the transition where thermal driving goes from negligible to comparable to line driving. We summarize a large set of hydrodynamical simulations of radial flows to illustrate how the wind properties change during the transition and the dependence of these properties on the assumed SED and governing flow parameters.