A disk wind in AB Aurigae traced with H alpha interferometry

作者：Perraut K^*; Dougados C; Lima G H R A; Benisty M; Mourard D; Ligi R; Nardetto N; Tallon Bosc I; ten Brummelaar T; Farrington C

来源：Astronomy & Astrophysics, 2016, 596: A17.

DOI：10.1051/0004-6361/201628931

• methods: observational
• techniques: high angular resolution
• techniques: interferometric
• circumstellar matter

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 alpha 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 alpha 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 alpha line, indicating that we clearly resolve the H alpha 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 alpha 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 degrees +/- 15 degrees 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 alpha 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 alpha 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 gamma line and confirm the importance of outflows in the HI line formation processes in young intermediate mass stars.

• 出版日期2016-12