  A crucial issue in star formation is to understand the physical mechanism by
which mass is accreted onto and ejected by a young star. The visible
spectrometer VEGA on the CHARA array can be an efficient means of probing the
structure and the kinematics of the hot circumstellar gas at sub-AU. For the
first time, we observed the Herbig Ae star AB Aur in the H$\alpha$ emission
line, using the VEGA low spectral resolution on two baselines of the array. We
computed and calibrated the spectral visibilities between 610 nm and 700 nm. To
simultaneously reproduce the line profile and the visibility, we used a 1-D
radiative transfer code that calculates level populations for hydrogen atoms in
a spherical geometry and synthetic spectro-interferometric observables. We
clearly resolved AB Aur in the H$\alpha$ line and in a part of the continuum,
even at the smallest baseline of 34 m. The small P-Cygni absorption feature is
indicative of an outflow but could not be explained by a spherical stellar wind
model. Instead, it favors a magneto-centrifugal X-disk or disk-wind geometry.
The fit of the spectral visibilities could not be accounted for by a wind
alone, so we considered a brightness asymmetry possibly caused by large-scale
nebulosity or by the known spiral structures, inducing a visibility modulation
around H$\alpha$. Thanks to the unique capabilities of VEGA, we managed to
simultaneously record for the first time a spectrum at a resolution of 1700 and
spectral visibilities in the visible range on a target as faint as $m_{V}$ =
7.1. It was possible to rule out a spherical geometry for the wind of AB Aur
and provide realistic solutions to account for the H$\alpha$ emission
compatible with magneto-centrifugal acceleration. The study illustrates the
advantages of optical interferometry and motivates observations of other bright
young stars to shed light on the accretion/ejection processes.
