The GRAVITY young stellar object survey ; The GRAVITY young stellar object survey: X. Probing the inner disk and magnetospheric accretion region of CI Tau

International audience ; Context. T Tauri stars are known to be the cradle of planet formation. Most exoplanets discovered to date lie at the very inner part of the circumstellar disk (<1 au). The innermost scale of young stellar objects is therefore a compelling region to be addressed, and long-baseline interferometry is a key technique to unveil their mysteries. Aims. We aim to spatially and spectrally resolve the innermost scale (≤1 au) of the young stellar system CI Tau to constrain the inner disk properties and better understand the magnetospheric accretion phenomenon. Methods. The high sensitivity offered by the combination of the four 8-m class telescopes of the Very Large Telescope Interferometer (VLTI) allied with the high spectral resolution (R ~ 4000) of the K -band beam combiner GRAVITY offers a unique capability to probe the sub-au scale of the CI Tau system, tracing both dust (continuum) and gas (Br γ line) emission regions. We developed a physically motivated geometrical model to fit the interferometric observables – visibilities and closure phases (CP) – and constrained the physical properties of the inner dusty disk. The continuum-corrected pure line visibilities have been used to estimate the size of the Hydrogen I Br γ emitting region. Results. From the K -band continuum study, we report a highly inclined ( i ~ 70°) resolved inner dusty disk, with an inner edge located at a distance of 21 ± 2 R ★ from the central star, which is significantly larger than the dust sublimation radius ( R sub = 4.3 to 8.6 R ★ ). The inner disk appears misaligned compared to the outer disk observed by ALMA and the non-zero closure phase indicates the presence of an asymmetry that could be reproduced with an azimuthally modulated ring with a brighter south-west side. From the differential visibilities across the Br γ line, we resolved the line-emitting region, and measured a size of 4.8 - 1.0 + 0.8 R ★ . Conclusions. The extended inner disk edge compared to the dust sublimation radius is consistent with the claim ...