The critical role of clumping in line-driven disc winds

TL;DR

This paper demonstrates that small-scale clumping in line-driven disc winds significantly enhances wind mass-loss rates and spectral features, resolving previous overionization issues and aligning theory with observations in white dwarfs and AGNs.

Contribution

The study introduces the microclumping approximation into simulations, showing how modest clumping factors can reconcile line-driven wind models with observed properties.

Findings

Clumping increases wind mass-loss rates substantially.

Clumpy models produce UV resonance lines absent in smooth models.

Clumping modifies the broad-band optical/UV spectral energy distribution.

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 $\dot{M}_{\rm wind}$ and yielding $\dot{M}_{\rm wind}/\dot{M}_{\rm acc}\!\gtrsim\!10^{-4}$ 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.