The magnetic field and accretion regime of CI Tau

TL;DR

This study uses spectropolarimetric data to analyze CI Tau's magnetic field and accretion processes, revealing a strong poloidal magnetic field that influences disc truncation and challenges previous planet detection claims.

Contribution

It provides the first detailed characterization of CI Tau's large-scale magnetic field and its role in star-disc interactions, with implications for planetary detection.

Findings

CI Tau has a 9-day rotation period and a strong, mainly poloidal magnetic field.

The magnetic field can evacuate the inner disc up to 50% of the co-rotation radius.

Stellar activity likely causes the observed radial velocity modulation, questioning the planet candidate.

Abstract

This paper exploits spectropolarimetric data of the classical T Tauri star CI Tau collected with ESPaDOnS at the Canada-France-Hawaii Telescope, with the aims of detecting and characterizing the large-scale magnetic field that the star hosts, and of investigating how the star interacts with the inner regions of its accretion disc through this field. Our data unambiguously show that CI Tau has a rotation period of 9.0d, and that it hosts a strong, mainly poloidal large-scale field. Accretion at the surface of the star concentrates within a bright high-latitude chromospheric region that spatially overlaps with a large dark photospheric spot, in which the radial magnetic field reaches -3.7kG. With a polar strength of -1.7kG, the dipole component of the large-scale field is able to evacuate the central regions of the disc up to about 50% of the co-rotation radius (at which the Keplerian orbital period equals the stellar rotation period) throughout our observations, during which the average accretion rate was found to be unusually high. We speculate that the magnetic field of CI Tau is strong enough to sustain most of the time a magnetospheric gap extending to at least 70% of the co-rotation radius, which would explain why the rotation period of CI Tau is as long as 9d. Our results also imply that the 9d radial velocity (RV) modulation that CI Tau exhibits is attributable to stellar activity, and thus that the existence of the candidate close-in massive planet CI Tau b to which these RV fluctuations were first attributed needs to be reassessed with new evidence.