The GRAVITY young stellar object survey -- XI. Probing the inner disk and magnetospheric accretion region of CI Tau

Aims: We aim at spatially and spectrally resolving the innermost scale of the young stellar object 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 telescopes of the VLTI allied with the spectral resolution 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 and gas emission regions. We develop a geometrical model to fit the interferometric observables and constrain the physical properties of the inner dusty disk. The continuum-corrected pure line visibilities have been used to estimate the size of the Br$\gamma$ emitting region. Results: From the K-band continuum study, we report an highly inclined resolved inner dusty disk, with an inner edge located at a distance of $21\pm2\,R_\star$ from the central star, which is significantly larger than the dust sublimation radius (R$_{sub}= 4.3$ to $8.6\,R_\star$). The inner disk appears misaligned compared to the outer disk observed by ALMA and the non-zero closure phase indicates the presence of a bright asymmetry on the south-west side. From the differential visibilities across the Br$\gamma$ line, we resolve the line emitting region, and measure a size of $4.8^{+0.8}_{-1.0}$ $R_\star$. Conclusions: The extended inner disk edge compared to the dust sublimation radius is consistent with the claim of an inner planet, CI Tau b, orbiting close-in. The inner-outer disk misalignment may be induced by gravitational torques or magnetic warping. The size of the Br$\gamma$ emitting region is consistent with the magnetospheric accretion process. Assuming it corresponds to the magnetospheric radius, it is significantly smaller than the co-rotation radius, which suggests an unstable accretion regime that is consistent with CI Tau being a burster.