Constraints on Clumps in the Representative Wind of the WN8 Wolf-Rayet star HD96548 = WR40 with Simultaneous Broadband Light and Linear-Polarisation Variability

TL;DR

This study presents simultaneous optical intensity and polarisation observations of the WN8 Wolf-Rayet star WR40, revealing uncorrelated variability and suggesting wind clumps emit via free-free processes, advancing understanding of stellar wind structures.

Contribution

First simultaneous broadband photometric and polarimetric observations of WR40, with a model explaining variability properties and wind clump emission mechanisms.

Findings

Photometric variability exceeds polarimetric variability by an order of magnitude.

No correlation between photometric and polarimetric variations.

Wind clumps emit through free-free processes in addition to scattering.

Abstract

We report precision ground-based broadband optical intensity and linear-polarisation light-curves for the sky's brightest WN8 star, WR40. WN8 stars are notorious for their high level of variability, stemming from stochastic clumps in their strong winds that are slower and less hot than the winds of most other Wolf-Rayet (WR) stars. We confirm previous results that many WR stars display an amplitude of variability that is an order-of-magnitude higher in photometry than in polarimetry. For the first time, the unique nature of near simultaneity of our photometric and polarimetric observations of WR40 allows us to check whether the two types of variability show correlated behaviour, of which we find none. Assuming simple temporal functions for the brightness and polarisation of individual clumps, a model for simulated light curves is found to reproduce the properties of the observations, specifically the absence of correlation between photometric and polarimetric variations, the ratio of standard deviations for photometric and polarimetric variability, and the ratio of the average intrinsic polarisation relative to its standard deviation. Mapping the solution for time variability to a spatial coordinate suggests that the wind clumps of WR40 are free-free emitting in addition to light scattering.