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

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 γ2 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+9-11°, 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.