The variability of the BRITE-est Wolf-Rayet binary, $\gamma^2$ Velorum I. Photometric and spectroscopic evidence for colliding winds

TL;DR

This study presents multi-color photometry and high-resolution spectroscopy of the Wolf-Rayet binary $ ext{γ}^2$ Velorum, revealing how colliding stellar winds influence observable properties and providing detailed simulations of wind interactions.

Contribution

It offers the first combined photometric and spectroscopic analysis of $ ext{γ}^2$ Velorum, including revised orbital parameters and hydrodynamical models of colliding winds.

Findings

Light curve and emission properties depend inversely on binary separation.

Phase-dependent excess emission is caused by line emission, not continuum changes.

Hydrodynamical simulations match observed wind shock features.

Abstract

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