  Young close binaries clear central cavities in their surrounding circumbinary
disk from which the stars can still accrete material. This process takes place
within the very first astronomical units, and is still not well constrained as
the observational evidence has been gathered, until now, only by means of
spectroscopy. The young object HD200775 (MWC361) is a massive spectroscopic
binary (separation of ~15.9mas, ~5.0~AU), with uncertain classification
(early/late Be), that shows a strong and variable Halpha emission. We aim to
study the mechanisms that produce the Halpha line at the AU-scale. Combining
the radial velocity measurements and astrometric data available in the
literature, we determined new orbital parameters. With the VEGA instrument on
the CHARA array, we spatially and spectrally resolved the Halpha emission of
HD200775, at low and medium spectral resolutions (R~1600 and 5000) over a full
orbital period (~3.6 years). We observe that the Halpha 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 Halpha emission (at 1 to 2-sigma level) and location (at 1 to 5-sigma
level). The spatial extent of the Halpha emission, as probed by a Gaussian
FWHM, is minimum at the ascending node (0.67+/-0.20 mas, i.e., 0.22+/-0.06 AU),
and more than doubles at 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 Halpha emission is due to accretion onto
the secondary. These findings, together with the wide Halpha line profile, may
be due to a non-spherical wind enhanced at periastron.

Solar and Stellar Astrophysics;Earth and Planetary Astrophysics