Probing the accretion-ejection connection with VLTI/AMBER High spectral resolution observations of the Herbig Ae star HD163296

Context. Accretion and ejection are tightly connected and represent the fundamental mechanisms regulating star formation. However, the exact physical processes involved are not yet fully understood. Aims. We present high angular and spectral resolution observations of the Br. emitting region in the Herbig Ae star HD163296 (MWC275) in order to probe the origin of this line and constrain the physical processes taking place at sub-AU scales in the circumstellar region. Methods. By means of VLTI-AMBER observations at high spectral resolution (R similar to 12 000), we studied interferometric visibilities, wavelength-differential phases, and closure phases across the Br. line of HD163296. To constrain the physical origin of the Br. line in Herbig Ae stars, all the interferometric observables were compared with the predictions of a line radiative transfer disc wind model. Results. The measured visibilities clearly increase within the Br gamma line, indicating that the Br. emitting region is more compact than the continuum. By fitting a geometric Gaussian model to the continuum-corrected Br. visibilities, we derived a compact radius of the Br. emitting region of similar to 0.07 +/- 0.02 AU (Gaussian half width at half maximum; or a ring-fit radius of similar to 0.08 +/- 0.02 AU). To interpret the observations, we developed a magneto-centrifugally driven disc wind model. Our best disc wind model is able to reproduce, within the errors, all the interferometric observables and it predicts a launching region with an outer radius of similar to 0.04 AU. However, the intensity distribution of the entire disc wind emitting region extends up to similar to 0.16 AU. Conclusions. Our observations, along with a detailed modelling of the Br. emitting region, suggest that most of the Br. emission in HD163296 originates from a disc wind with a launching region that is over five times more compact than previous estimates of the continuum dust rim radius.