Rapid polarization variations in the O4 supergiant ζ Puppis

We present time-series linear-polarization observations of the bright O4 supergiant zeta Puppis. The star is found to show polarization variation on time-scales of around an hour and longer. Many of the observations were obtained contemporaneously with Transiting Exoplanet Survey Satellite (TESS) photometry. We find that the polarization varies on similar time-scales to those seen in the TESS light curve. The previously reported 1.78-d photometric periodicity is seen in both the TESS and polarization data. The amplitude ratio of photometry to polarization is similar to 9 for the periodic component and the polarization variation is oriented along position angle similar to 70 degrees-160 degrees. Higher frequency stochastic variability is also seen in both data sets with an amplitude ratio of similar to 19 and no preferred direction. We model the polarization expected for a rotating star with bright photospheric spots and find that models that fit the photometric variation produce too little polarization variation to explain the observations. We suggest that the variable polarization is more likely the result of scattering from the wind, with corotating interaction regions producing the periodic variation and a clumpy outflow producing the stochastic component. The H alpha emission line strength was seen to increase by 10 per cent in 2021 with subsequent observations showing a return to the pre-2018 level.