The variability of the BRITE-est Wolf–Rayet binary, γ2 Velorum–I. Photometric and spectroscopic evidence for colliding winds

Abstract

We report on the first multi-color precision light curve of the bright\nWolf-Rayet binary $\\gamma^2$ Velorum, obtained over six months with the\nnanosatellites in the BRITE- Constellation fleet. In parallel, we obtained 488\nhigh-resolution optical spectra of the system. In this first report on the\ndatasets, we revise the spectroscopic orbit and report on the bulk properties\nof the colliding winds. We find a dependence of both the light curve and excess\nemission properties that scales with the inverse of the binary separation. When\nanalyzing the spectroscopic properties in combination with the photometry, we\nfind that the phase dependence is caused only by excess emission in the lines,\nand not from a changing continuum. We also detect a narrow, high-velocity\nabsorption component from the He I $\\lambda$5876 transition, which appears\ntwice in the orbit. We calculate smoothed-particle hydrodynamical simulations\nof the colliding winds and can accurately associate the absorption from He I to\nthe leading and trailing arms of the wind shock cone passing tangentially\nthrough our line of sight. The simulations also explain the general strength\nand kinematics of the emission excess observed in wind lines such as C III\n$\\lambda$5696 of the system. These results represent the first in a series of\ninvestigations into the winds and properties of $\\gamma^2$ Velorum through\nmulti-technique and multi-wavelength observational campaigns.\n