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

TL;DR

This study spatially and spectrally resolves the innermost regions of CI Tau, revealing an inclined inner dusty disk, a compact Brγ emission zone, and insights into magnetospheric accretion and disk misalignment.

Contribution

It provides the first high-resolution interferometric measurements of CI Tau's inner disk and magnetospheric region, revealing disk structure and accretion characteristics.

Findings

Inner dusty disk is highly inclined and larger than dust sublimation radius.

The Brγ emission region is compact, consistent with magnetospheric accretion.

Inner disk misalignment suggests gravitational or magnetic influences.

Abstract

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.