The critical role of clumping in line-driven disc winds

Abstract

Radiation pressure on spectral lines is a promising mechanism for powering disc winds from accreting white dwarfs (AWDs) and active galactic nuclei (AGN). However, in radiation-hydrodynamic simulations, overionization reduces line opacity and quenches the line force, which suppresses outflows. Here, we show that small-scale clumping can resolve this problem. Adopting the microclumping approximation, our new simulations demonstrate that even modest volume filling factors ($f_V\sim 0.1-0.01$) can dramatically increase the wind mass-loss rate by lowering its ionization state -- raising ${\dot{M}}_{\mathrm{wind}}$ and yielding ${\dot{M}}_{\mathrm{wind}}/{\dot{M}}_{\mathrm{acc}}\gtrsim 10^{-4}$ for such modest filling factors. Clumpy wind models produce the UV resonance lines that are absent from smooth wind models. They can also reprocess a significant fraction of the disc luminosity and thus dramatically modify the broad-band optical/UV SED. Given that theory and observations indicate that disc winds are intrinsically inhomogeneous, clumping offers a physically motivated solution. Together, these results provide the first robust, self-consistent demonstration that clumping can reconcile line-driven wind theory with observations across AWDs and AGNs.

9 pages, 5 figures, 1 table, accepted for publication in MNRAS