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Context. A crucial issue in star formation is understanding the physical mechanism by which mass is accreted onto and ejected by a young star, then collimated into jets. Hydrogen lines are often used to trace mass accretion in young stars, but recent observations suggest that they could instead trace mass outflow in a disk wind. Aims. Obtaining direct constraints on the HI line formation regions is crucial in order to disentangle the different models. We present high angular and spectral resolution observations of the H α line of the Herbig Ae star AB Aur to probe the origin of this line at sub-AU scales, and to place constraints on the geometry of the emitting region. Methods. We use the visible spectrograph VEGA at the CHARA long-baseline optical array to resolve the AB Aur circumstellar environment from spectrally resolved interferometric measurements across the H α emission line. We developed a 2D radiative transfer model to fit the emission line profile and the spectro-interferometric observables. The model includes the combination of a Blandford & Payne magneto-centrifugal disk wind and a magnetospheric accretion flow. Results. We measure a visibility decrease within the H α line, indicating that we clearly resolve the H α formation region. We derive a Gaussian half width at half maximum between 0.05 and 0.15 AU in the core of the line, which indicates that the bulk of the H α emission has a size scale intermediate between the disk inner truncation radius and the dusty disk inner rim. A clear asymmetric differential phase signal is found with a minimum of −30° ± 15° towards the core of the line. We show that these observations are in general agreement with predictions from a magneto-centrifugal disk wind arising from the innermost regions of the disk. Better agreement, in particular with the differential phases, is found when a compact magnetospheric accretion flow is included. Conclusions. We resolve the H α formation region in a young accreting intermediate mass star and show that both the spectroscopic and interferometric measurements can be reproduced well by a model where the bulk of H α forms in a MHD disk wind arising from the innermost regions of the accretion disk. These findings support similar results recently obtained in the Br γ line and confirm the importance of outflows in the HI line formation processes in young intermediate mass stars.

A disk wind in AB Aurigae traced with Hαinterferometry