Description
Disk winds are prevalent in a variety of accreting systems, such as young stellar objects, accreting white dwarfs (AWDs), X-ray binaries, and active galactic nuclei (AGN). In the context of AWDs and AGN, radiation pressure acting on spectral lines has been proposed as a key mechanism for driving these outflows. Although previous hydrodynamical simulations have largely supported this idea, they relied on highly approximate, quasi-1D treatments of ionization and radiative transfer. Here, we present state-of-the-art numerical simulations based on realistic radiation-hydrodynamical (RHD) calculations of line-driven disk winds. Our 2.5D RHD code includes a detailed treatment of frequency-dependent radiative transfer through the wind, the corresponding ionization state, and the resulting radiative accelerations. Applying this method to AWDs, we find that it is far more difficult to power the outflows observed in these systems via line driving than previously thought. Physically, this occurs because the winds are far more prone to over-ionization than suggested by quasi-1D treatments. The same issue also appears to affect AGN, where even moderate X-ray fluxes can overionize the outflow. While these findings challenge the viability of line-driven winds under standard assumptions, observations demonstrate that real disk winds do avoid overionization. This discrepancy reveals a critical gap in our physical understanding of line-driven disk winds. The missing key ingredient(s) may include wind clumping, magnetic fields, or non-standard spectral energy distributions.
