Long-period modulation of the classical T Tauri star CI Tau

Abstract

Context. Detecting planets within protoplanetary disks around young stars is essential for understanding planet formation and evolution. However, planet detection using the radial velocity method faces challenges due to the strong stellar activity in these early stages. Aims. We detect long-term periodicities in photometric and spectroscopic time series of the classical T Tauri star (CTTS) CI Tau, and retrieve evidence for inner embedded planets in its disk. Methods. The study conducted photometric and spectroscopic analyses using K2 and Las Cumbres Observatory Global Network light curves, and high-resolution spectra from ESPaDOnS and SPIRou. We focused our radial velocity analysis on a wavelength domain less affected by spot activity. To account for spot effects, a quasi-periodic Gaussian process model was applied to K2 light curve, ESPaDOnS, and SPIRou radial velocity data. Additionally, a detailed bisector analysis on cross-correlation functions was carried out to understand the cause of long-term periodicity. Results. We detect coherent periods at ~6.6d, 9d, ~11.5d, ~14.2d, and ~25.2d, the last of which is seen consistently across all datasets. Bisector analysis of the cross-correlation functions provides strong hints for combined activity-induced and Doppler reflex signals in the radial velocities at a period of 25.2 d. Our analysis suggests that this periodicity is best explained by the presence of a 3.6 ± 0.3 MJup eccentric (e ~ 0.58) planet at a semi-major axis of 0.17 au. Conclusions. We report the detection of a massive inner planet in CI Tau. Our study outlines the difficulty of searching for disk-embedded planets in the inner 0.1 au of young and active systems. When searching for planets in actively accreting stars such as CI Tau, we demonstrate that the primary limitation is stellar activity rather than the precision of RV measurements provided by the instrument.