Probing the magnetospheric accretion region of the young pre-transitional disk system DoAr 44 using VLTI/GRAVITY

J Bouvier,K Perraut,M Benisty,C Dougados,G Duvert,J.-B Le Bouquin,J.-P Berger,W Brandner,E Alécian

doi:10.1051/0004-6361/202037611

Abstract

Context. Young stellar objects are thought to accrete material from their circumstellar disks through their strong stellar magnetospheres. Aims. We aim to directly probe the magnetospheric accretion region on a scale of a few 0.01 au in a young stellar system using long-baseline optical interferometry. Methods. We observed the pre-transitional disk system DoAr 44 with VLTI/GRAVITY on two consecutive nights in the K-band. We computed interferometric visibilities and phases in the continuum and in the Brγ line in order to constrain the extent and geometry of the emitting regions. Results. We resolve the continuum emission of the inner dusty disk and measure a half-flux radius of 0.14 au. We derive the inclination and position angle of the inner disk, which provides direct evidence that the inner and outer disks are misaligned in this pre-transitional system. This may account for the shadows previously detected in the outer disk. We show that Brγ emission arises from an even more compact region than the inner disk, with an upper limit of 0.047 au (~5 R⋆). Differential phase measurements between the Brγ line and the continuum allow us to measure the astrometric displacement of the Brγ line-emitting region relative to the continuum on a scale of a few tens of microarcsec, corresponding to a fraction of the stellar radius. Conclusions. Our results can be accounted for by a simple geometric model where the Brγ line emission arises from a compact region interior to the inner disk edge, on a scale of a few stellar radii, fully consistent with the concept of magnetospheric accretion process in low-mass young stellar systems.

Highlights

During the first few million years, the young low-mass stellar systems known as T Tauri stars are surrounded by a circumstellar disk from which they accrete material
We report the data reduction processes that allowed us to measure interferometric visibilities and phases along the six baselines sampled by the four ESO/VLTI unit telescopes
Since the error bars provided by the pipeline can be underestimated and/or do not include the residual calibration effects, we adopted conservative error bars: for the squared visibilities, we computed the rms over exposures probing the same spatial frequencies, which yields error bars amounting to 2% in all spectral channels; for the closure phases, we computed the rms over the 36 exposures recorded during the two nights and derived an rms of 0.8◦ when considering all the spectral channels, and of 0.3◦ when considering the central spectral channel only

Summary

Introduction

During the first few million years, the young low-mass stellar systems known as T Tauri stars are surrounded by a circumstellar disk from which they accrete material. At the distance of the nearest star forming regions (≥100 pc), the angular size of the star-disk interaction region is of the order of a milli-arcsec (mas) or less, a scale barely reachable with even the largest telescopes. This is the reason why the magnetospheric accretion process in T Tauri stars has been investigated mostly indirectly so far, for example, by monitoring the variability of the inner system as it rotates (e.g., Bouvier et al 2007b; Alencar et al 2012, 2018; Donati et al 2019).

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