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A crucial issue in star formation is to understand the physical mechanism bywhich mass is accreted onto and ejected by a young star. The visiblespectrometer VEGA on the CHARA array can be an efficient means of probing thestructure and the kinematics of the hot circumstellar gas at sub-AU. For thefirst time, we observed the Herbig Ae star AB Aur in the H$\alpha$ emissionline, using the VEGA low spectral resolution on two baselines of the array. Wecomputed and calibrated the spectral visibilities between 610 nm and 700 nm. Tosimultaneously reproduce the line profile and the visibility, we used a 1-Dradiative transfer code that calculates level populations for hydrogen atoms ina spherical geometry and synthetic spectro-interferometric observables. Weclearly 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 isindicative of an outflow but could not be explained by a spherical stellar windmodel. 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 windalone, so we considered a brightness asymmetry possibly caused by large-scalenebulosity or by the known spiral structures, inducing a visibility modulationaround H$\alpha$. Thanks to the unique capabilities of VEGA, we managed tosimultaneously record for the first time a spectrum at a resolution of 1700 andspectral 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 Aurand provide realistic solutions to account for the H$\alpha$ emissioncompatible with magneto-centrifugal acceleration. The study illustrates theadvantages of optical interferometry and motivates observations of other brightyoung stars to shed light on the accretion/ejection processes.

The H$\alpha$ line forming region of AB Aur spatially resolved at sub-AU with the VEGA/CHARA spectro-interferometer