The H α line forming region of AB Aurigae spatially resolved at sub-AU with the VEGA/CHARA spectro-interferometer

Context. A crucial issue in star formation is to understand the physical mechanism by which mass is accreted onto and ejected bya young star. To derive key constraints on the launching point of the jets and on the geometry of the winds, the visible spectro-polarimeter VEGA installed on the CHARA optical array can be an efficient means of probing the structure and the kinematics of thehot circumstellar gas at sub-AU.Aims. For the first time, we observed the Herbig Ae star AB Aur in the Hα emission line, using the VEGA low spectral resolution(R = 1700) on two baselines of the array.Methods. We computed and calibrated the spectral visibilities of AB Aur between 610 nm and 700 nm in spectral bands of20.4 nm. To simultaneously reproduce the line profile and the inferred visibility around Hα, we used a 1D radiative transfer code(RAMIDUS/PROFILER) that calculates level populations for hydrogen atoms in a spherical geometry and that produces syntheticspectro-interferometric observables.Results. We clearly resolved AB Aur in the Hα line and in a part of the continuum, even at the smallest baseline of 34 m. The smallP-Cygni absorption feature is indicative of an outflow but could not be explained by a spherical stellar wind model. Instead, it favorsa magneto-centrifugal X-disk or disk-wind geometry. The fit of the spectral visibilities from 610 to 700 nm could not be accountedfor by a wind alone, so another component inducing a visibility modulation around Hα needed to be considered. We thus considereda brightness asymmetry possibly caused by large-scale nebulosity or by the known spiral structures.Conclusions. Thanks to the unique capabilities of VEGA, we managed to simultaneously record for the first time a spectrum at aresolution of 1700 and spectral visibilities in the visible range on a target as faint as mV = 7.1. It was possible to rule out a sphericalgeometry for the wind of AB Aur and provide realistic solutions to account for the Hα emission compatible with magneto-centrifugalacceleration. It was difficult, however, to determine the exact morphology of the wind because of the surrounding asymmetric ne-bulosity. The study illustrates the advantages of optical interferometry and motivates observations of other bright young stars in thesame way to shed light on the accretion/ejection processes.