Direct constraint on the distance of γ² Velorum from AMBER/VLTI observations

Abstract

Context. Interferometry can provide spatially resolved observations of massive star binary systems and their colliding winds, which thus far have been studied mostly with spatially unresolved observations. Aims. We present the first AMBER/VLTI observations, taken at orbital phase 0.32, of the Wolf-Rayet and O (WR+O) star binary system γ 2 Velorum and use the interferometric observables to constrain its properties. Methods. The AMBER/VLTI instrument was used with the telescopes UT2, UT3, and UT4 on baselines ranging from 46 m to 85 m. It delivered spectrally dispersed visibilities, as well as differential and closure phases, with a resolution R = 1500 in the spectral band 1.95−2.17 µm. We interpret these data in the context of a binary system with unresolved components, neglecting in a first approximation the wind-wind collision zone flux contribution. Results. Using WR-and O-star synthetic spectra, we show that the AMBER/VLTI observables result primarily from the contribution of the individual components of the WR+O binary system. We discuss several interpretations of the residuals, and speculate on the detection of an additional continuum component, originating from the free-free emission associated with the wind-wind collision zone (WWCZ), and contributing at most to the observed K-band flux at the 5% level. Based on the accurate spectroscopic orbit and the Hipparcos distance, the expected absolute separation and position angle at the time of observations were 5.1 ± 0.9 mas and 66 ± 15• , respectively. However, using theoretical estimates for the spatial extent of both continuum and line emission from each component, we infer a separation of 3.62 +0.11 −0.30 mas and a position angle of 73• , compatible with the expected one. Our analysis thus implies that the binary system lies at a distance of 368 +38 −13 pc, in agreement with recent spectrophotometric estimates, but significantly larger than the Hipparcos value of 258 +41 −31 pc.

VEGA: Visible spEctroGraph and polArimeter for the CHARA array: principle and performance

“…Baselines ranging from 100 m to 300 m are mandatory to provide a detectable effect that could be measured accurately for the closest WR stars. SUSI ) and the VLTI (Glindemann et al 2004) have already carried out pioneering works in this field (North et al 2007;Millour et al 2007). …”

Binaries and Multiple Systems Observed with the CHARA, NPOI, SUSI and VLTI Interferometric Eyes

“…This system was observed by Millour et al (2007) with the VLTI/AMBER and by North et al (2007) with the SUSI interferometer. Millour et al (2007) using a relatively restrained data set have set tight constraints on the geometrical parameters of the γ 2 Velorum orbit.…”

“…This system was observed by Millour et al (2007) with the VLTI/AMBER and by North et al (2007) with the SUSI interferometer. Millour et al (2007) using a relatively restrained data set have set tight constraints on the geometrical parameters of the γ 2 Velorum orbit. A nice example of the constraints brought by interferometry is the work done by Millour et al (2007) where they have drastically reduced the error box derived from the classical radial velocity method by using an interferometric fit to retrieve the geometrical parameters of the binary star.…”

“…Millour et al (2007) using a relatively restrained data set have set tight constraints on the geometrical parameters of the γ 2 Velorum orbit. A nice example of the constraints brought by interferometry is the work done by Millour et al (2007) where they have drastically reduced the error box derived from the classical radial velocity method by using an interferometric fit to retrieve the geometrical parameters of the binary star. Moreover, the smaller separation measured by interferometry means that the distance of the system must be reevaluated to 368 pc, in agreement with recent spectrophotometric estimates, but significantly larger than the Hipparcos value of 258 pc.…”

An interferometric journey around massive stars

“…a spiral-like pattern tracing the orbital motion of the wind-collision layer. The first Wolf-Rayet stars observed with long-baseline optical interferometry were γ 2 Vel (Millour et al 2007) and WR 118 (Millour et al 2009b). In both papers, the authors manage to resolve the system, and in that second one, they found closure phase signal compatible with a pinwheel model.…”