Polarization Variability Arising from Clumps in the Winds of Wolf-Rayet Stars

TL;DR

This study models the polarimetric and photometric variability of Wolf-Rayet stars caused by wind clumps, improving previous models by including radial expansion and stellar occultation effects, and constrains wind parameters through observational data fitting.

Contribution

The paper introduces an improved model for WR star wind clumps that accounts for radial expansion and uses Monte Carlo simulations to constrain wind parameters from observations.

Findings

Clump velocity law index beta is well constrained.

Ejection rate of clumps is supported by spectral features.

Fraction of mass loss in clumps matches observational polarization.

Abstract

The polarimetric and photometric variability of Wolf-Rayet (WR) stars as caused by clumps in the winds, is revisited. In the model which is improved from Li et al. 2000, the radial expansion of the thickness is accounted for, but we retain the dependence on the beta velocity law, stellar occultation effects. We again search for parameters that can yield results consistent with observations in regards to the mean polarization, the ratio of polarimetric to photometric variability, and the volume filling factor. Clump generation and spatial distribution are randomized by the Monte Carlo method so as to produce clumps which are, in the mean, distributed uniformly in space and have time intervals with a Gaussian distribution. The generated clumps move radially outward with a velocity law determined by a beta index, and the angular size of the clumps is assumed to keep fixed. By fitting the observational results and the volume filling factor, the clump velocity law index beta and clump ejection rate are inferred, and are found to be well constrained. In addition, the subpeak features on broad emission lines seem to support the clump ejection rate. Meanwhile, the fraction of the total mass loss rate that is contained in the clumps is obtained by fitting the observed polarization. We conclude that this picture for the clump properties produces a valuable diagnostic of WR wind structure.