BRITE Nanosat Photometry Reveals Bright Spots in the Single Wolf–Rayet Star EZ CMa That Can Drive Spectral-line-emitting Corotating Interaction Regions

Abstract We analyze an unprecedented 4.5 contiguous months of 2015–2016 BRITE nanosatellite precision optical photometry of the 7th mag Wolf–Rayet (WR) star EZ CMa = WR 6 = HD 50896. A Fourier transform (FT) of the light curve (LC) reveals a period of 3.751 ± 0.011 days (broadly consistent with all previous data and analyses), with five other harmonics at simple multiples of the fundamental frequency. All harmonics are used to determine the best period. The FT shows Lorentzian as opposed to Gaussian peaks that were a first hint that EZ CMa was behaving oddly. A more revealing 3D dynamical plot (rotation-cycle number versus rotation phase and color-coded intensity) of the LC further shows at least a dozen apparently independent bright spots over ∼35 rotations, each randomly starting in time and stellar azimuth and lasting FWHM ∼ 5 rotations, but always following the same rotation period. This randomness goes against any binary hypothesis. A model simulation suggests that we are likely seeing the bright rotating footprints of corotating interaction regions (CIRs). The model bright spots must be located above the hydrostatic radius to be compatible with the observed LC. While EZ CMa remains the key WR star showing this scenario, there are now five other WR stars claimed in the literature with CIRs, and more are likely to follow. This discovery opens the door to searching for local magnetic fields that may be needed to stabilize the bright spots.