The chaotic wind of WR 40 as probed by BRITE

TL;DR

This study investigates the complex, stochastic wind variability of Wolf-Rayet star WR 40 using BRITE satellite photometry, proposing a clumpy wind model with power-law size distribution that explains observed light fluctuations.

Contribution

It introduces a stochastic clumpy wind model with power-law size distribution to explain WR 40's photometric variability, supported by simulated light curves matching observations.

Findings

Light variability dominated by transient, low-frequency signals.

Clump size distribution likely follows a negative power-law.

Simulated light curves resemble observed data well.

Abstract

Among Wolf-Rayet stars, those of subtype WN8 are the intrinsically most variable. We have explored the long-term photometric variability of the brightest known WN8 star, WR 40, through four contiguous months of time-resolved, single-passband optical photometry with the BRIght Target Explorer (BRITE) nanosatellite mission. The Fourier transform of the observed light-curve reveals that the strong light variability exhibited by WR 40 is dominated by many randomly-triggered, transient, low-frequency signals. We establish a model in which the whole wind consists of stochastic clumps following an outflow visibility promptly rising to peak brightness upon clump emergence from the optically thick pseudo-photosphere in the wind, followed by a gradual decay according to the right-half of a Gaussian. Free electrons in each clump scatter continuum light from the star. We explore a scenario where the clump size follows a power-law distribution, and another one with an ensemble of clumps of constant size. Both scenarios yield simulated light curves morphologically resembling the observed light curve remarkably well, indicating that one cannot uniquely constrain the details of clump size distribution with only a photometric light curve. Nevertheless, independent evidence favours a negative-index power law, as seen in many other astrophysical turbulent media.