2013A&A...555A.113B

Young close binaries clear central cavities in their surrounding circumbinary disk from which the stellar objects can still accrete material. This process takes place within the first astronomical unit and is still not well constrained because the observational evidence has been gathered, until now, only by means of spectroscopy. Theoretical models for T Tauri stars in close binaries predict a variability of the hydrogen emission lines attributable to periodic changes in the accretion rates as the secondary approaches periastron. Whether a similar scenario applies to more massive objects is unclear, and still needs to be proven observationally. The young object HD 200775 (MWC 361) is a massive spectroscopic binary (separation of ∼15.9mas, ∼5.0AU), with uncertain classification (early/late Be), that shows a strong and variable H_α emission. We aim to study the mechanisms that produce the H_α line at the AU-scale, and their dependence on binarity. Combining the radial velocity measurements and astrometric data available in the literature, we determined new orbital parameters and revised the distance to 320±51pc. With the VEGA instrument on the CHARA array, we spatially and spectrally resolved the H_α emission of HD 200775 on a scale of a few milliarcseconds, at low and medium spectral resolutions (R∼1600 and 5000). Our observations cover a single orbital period (∼3.6 years). Spectra, spectral visibilities, and differential phases have been derived. A simple analytical model of a face-on Gaussian located along the binary axis was used to analyze the interferometric observables over the spectral range. We observe that the H_α equivalent width varies with the orbital phase, and increases close to periastron, as expected from theoretical models that predict an increase of the mass transfer from the circumbinary disk to the primary disk. In addition, using spectral visibilities and differential phases, we find marginal variations of the typical extent of the H_α emission (at 1 to 2σ level) and location (at 1 to 5σ level). The spatial extent of the H_α emission, as probed by the Gaussian FWHM, is minimum at the ascending node (0.67±0.20mas, i.e., 0.22±0.06AU), and more than doubles at the periastron. In addition, the Gaussian photocenter is slightly displaced in the direction opposite to the secondary, ruling out the scenario in which all or most of the H_α emission is due to accretion onto the secondary. This favors a scenario in which the primary is responsible for the enhanced H_α activity at periastron. These findings, together with the wide H_α line profile, may be due to a non-spherical wind enhanced at periastron. For the first time in a system of this kind, we spatially resolve the H_α line and estimate that it is emitted in a region larger than the one usually inferred in accretion processes. The H_α line could be emitted in a stellar or disk-wind, enhanced at periastron as a result of gravitational perturbation, after a period of increased mass accretion rate. Our results suggest a strong connection between accretion and ejection in these massive objects, consistent with the predictions for lower-mass close binaries.