Numerical simulations and infrared spectro-interferometry reveal the wind collision region in γ2 Velorum

Abstract

Colliding stellar winds in massive binary systems have been studied through their radio, optical lines and strong X-ray emission for decades. More recently, near-infrared spectrointerferometric observations have become available in a few systems, but isolating the contribution from the individual stars and the wind collision region still remains a challenge. In this paper, we study the colliding wind binary γ 2 Velorum and aim at identifying the wind collision zone from infrared interferometric data, which provide unique spatial information to determine the wind properties. Our analysis is based on multi-epoch VLTI/AMBER data that allows us to separate the spectral components of both stars. First, we determine the astrometric solution of the binary and confirm previous distance measurements. We then analyse the spectra of the individual stars, showing that the O star spectrum is peculiar within its class. Then, we perform three-dimensional hydrodynamic simulations of the system from which we extract model images, visibility curves and closure phases which can be directly compared with the observed data. The hydrodynamic simulations reveal the 3D spiral structure of the wind collision region, which results in phase-dependent emission maps. Our model visibility curves and closure phases provide a good match when the wind collision region accounts for 3-10 percent γ 2 Vel's total flux in the near infrared. The dialogue between hydrodynamic simulations, radiative transfer models and observations allows us to fully exploit the observations. Similar efforts will be crucial to study circumstellar environments with the new generation of VLTI instruments like GRAVITY and MATISSE. † Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 60.A-9054, 074.A-9025,

Investigation of the WR 11 field at decimeter wavelengths

“…Second, the extension of the high optical depth zone is much larger than the typical dimension of the full binary system. According to Lamberts et al (2017), the semimajor axis of the orbit is about 3.5 milli-arseconds, which translates to a linear semimajor axis of about 250 R at a distance of about 340 pc (in agreement with the results published by North et al 2007). This is much smaller than the expected size of the radio photosphere at our selected wavelength, which is much larger than a few 1000 R at all frequencies below 300 GHz (Fig.…”

“…For an O7.5 spectral type, according to the calibra-tion of O-type star parameters published by Martins et al (2005), this points to a giant luminosity class. We note, however, that Lamberts et al (2017) suggest that the best-matching IR spectrum is that of an O6.5I star, but such a spectral classification would be at odd with the mass of about 28 solar masses which should be quite robust. In addition, let us caution that the Otype spectrum analyzed by Lamberts et al (2017) is very likely contaminated by some emission from the colliding-wind region.…”

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

“…Unfortunately, the only interferometric observation with spectral dispersion for γ 2 Vel was this single point from VLTI obtained during the commissioning phase of the instrument. Lamberts et al (2017) examined new VLTI/AMBER data and used numerical simulations to derive an orbit that was nearly identical to that reported by North et al (2007). Their numerical simulations allowed them to discern the effect of the spatially-resolved colliding winds' shock cone in the near-infrared continuum and in emission lines.…”

The First Dynamical Mass Determination of a Nitrogen-rich Wolf–Rayet Star Using a Combined Visual and Spectroscopic Orbit

“…Finally, we note that there are three well-established masses for WR stars, where the mass comes from a combination of a visual (interferometric) orbit and a doublelined spectroscopic orbit. The two additional systems are γ 2 Velorum (Lamberts et al 2017) which contains a WC8 star with a mass of 8.9 M , and WR 140 (Thomas et al, submitted) which contains a WC7 star with a mass of 10.3 M . It is interesting to note that the two WC stars appear to have similar masses to the WN star in WR 133.…”