Line-drag damping of Alfv\'en waves in radiatively driven winds of magnetic massive stars

TL;DR

This paper analyzes how Alfvén waves are damped in magnetized, line-driven stellar winds of massive stars, revealing that short-wavelength waves are strongly damped by line-drag effects, which impacts wind dynamics and observational diagnostics.

Contribution

It provides the first analytic linear stability analysis of magneto-radiative waves in magnetic, line-driven stellar winds, including line-scattering effects and wave coupling.

Findings

Short-wavelength Alfvén waves are strongly damped by line-drag effects.

Long-wavelength waves are decoupled from the radiation field.

Damping influences wind dynamics and observational diagnostics.

Abstract

Line-driven stellar winds from massive (OB) stars are subject to a strong line-deshadowing instability. Recently, spectropolarimetric surveys have collected ample evidence that a subset of Galactic massive stars hosts strong surface magnetic fields. We investigate here the propagation and stability of magneto-radiative waves in such a magnetised, line-driven wind. Our analytic, linear stability analysis includes line-scattering from the stellar radiation, and accounts for both radial and non-radial perturbations. We establish a bridging law for arbitrary perturbation wavelength after which we analyse separately the long- and short-wavelength limits. While long-wavelength radiative and magnetic waves are found to be completely decoupled, a key result is that short-wavelength, radially propagating Alfv\'en waves couple to the scattered radiation field and are strongly damped due to the line-drag effect. This damping of magnetic waves in a scattering-line-driven flow could have important effects on regulating the non-linear wind dynamics, and so might also have strong influence on observational diagnostics of the wind structure and clumping of magnetic line-driven winds.