Clumping in the inner winds of hot, massive stars from hydrodynamical line-driven instability simulations

TL;DR

This study demonstrates that including limb-darkening and photospheric perturbations in simulations of line-driven stellar winds leads to earlier wind structure formation near the star's surface, aligning better with observations of O-type stars.

Contribution

It introduces a linear perturbation analysis incorporating limb-darkening and photospheric perturbations, showing their significant impact on wind instability development.

Findings

Wind structure develops closer to the photosphere ($r \,\la\ 1.1 R_\star$) with limb-darkening included.

Simulations align better with observations of strong clumping near the wind base in O-type stars.

Limb-darkening reduces the stabilizing effect of diffuse radiation, enhancing instability growth.

Abstract

We investigate the effects of stellar limb-darkening and photospheric perturbations for the onset of wind structure arising from the strong, intrinsic line-deshadowing instability (LDI) of a line-driven stellar wind. A linear perturbation analysis shows that including limb-darkening reduces the stabilizing effect of the diffuse radiation, leading to a net instability growth rate even at the wind base. Numerical radiation-hydrodynamics simulations of the non-linear evolution of this instability then show that, in comparison with previous models assuming a uniformly bright star without base perturbations, wind structure now develops much closer ($r \la 1.1 R_\star$) to the photosphere. This is in much better agreement with observations of O-type stars, which typically indicate the presence of strong clumping quite near the wind base.