BRITE-Constellation high-precision time-dependent photometry of the early O-type supergiant ζ Puppis unveils the photospheric drivers of its small- and large-scale wind structures

Abstract

From $5.5$ months of dual-band optical photometric monitoring at the $1$ mmag\nlevel, BRITE-Constellation has revealed two simultaneous types of variability\nin the O4I(n)fp star $\\zeta$ Puppis: one single periodic non-sinusoidal\ncomponent superimposed on a stochastic component. The monoperiodic component is\nthe $1.78$ d signal previously detected by Coriolis/SMEI, but this time along\nwith a prominent first harmonic. The shape of this signal changes over time, a\nbehaviour that is incompatible with stellar oscillations but consistent with\nrotational modulation arising from evolving bright surface inhomogeneities. By\nmeans of a constrained non-linear light curve inversion algorithm we mapped the\nlocations of the bright surface spots and traced their evolution. Our\nsimultaneous ground-based multi-site spectroscopic monitoring of the star\nunveiled cyclical modulation of its He II $\\lambda4686$ wind emission line with\nthe $1.78$-day rotation period, showing signatures of Corotating Interaction\nRegions (CIRs) that turn out to be driven by the bright photospheric spots\nobserved by BRITE. Traces of wind clumps are also observed in the He II\n$\\lambda4686$ line and are correlated with the amplitudes of the stochastic\ncomponent of the light variations probed by BRITE at the photosphere,\nsuggesting that the BRITE observations additionally unveiled the photospheric\ndrivers of wind clumps in $\\zeta$ Pup and that the clumping phenomenon starts\nat the very base of the wind. The origins of both the bright surface\ninhomogeneities and the stochastic light variations remain unknown, but a\nsubsurface convective zone might play an important role in the generation of\nthese two types of photospheric variability.\n