Probing the magnetospheric accretion region of the young system DoAr 44 using VLTI/GRAVITY

Young stars accrete material from their circumstellar disk through magnetically-controlled funnel flows. This so-called magnetospheric accretion process is central to the properties of young systems and not yet fully deciphered. We report here on an attempt to directly probe the star-disk interaction region a scale of less than 0.1 au in the young system DoAr 44, using with VLTI/GRAVITY in the K-band. We computed interferometric visibilities and phases in the continuum and in the Br$\gamma$ line in order to constrain the extent and geometry of the emitting regions. We resolve the continuum emission of the inner dusty disk and measure a half-flux radius of 0.14 au. We show that Br$\gamma$ emission arises from an even more compact region than the inner disk, with an upper limit of 0.047 au (∼5 R$_\star$). Differential phase measurements between the Br$\gamma$ line and the continuum allow us to measure the astrometric displacement of the Br$\gamma$ line-emitting region relative to the continuum on a scale of a few tens of microarcsec, corresponding to a fraction of the stellar radius. Our results can be accounted for by a simple geometric model where the Br$\gamma$ 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.