Variable X-ray Emission of the Planet Hosting T Tauri Star CI Tau

TL;DR

CI Tau is a young, magnetically active T Tauri star hosting a candidate close-in planet. The authors use four Chandra ACIS-S exposures totaling 65 ks to characterize X-ray variability, discovering two quiescent phases around $kT \\approx 2\\,\\mathrm{keV}$ with $\\log L_x \\approx 29.74$ and two later phases with $\\;kT \\approx 4-5\\,\\mathrm{keV}$ and $\\log L_x \\approx 30.5$, consistent with magnetic activity-driven coronae. Comparison with XMM-Newton data from 2005 shows a historically cooler and hotter thermal components with $\\log L_x \\approx 29.64$, supporting a corona-dominated X-ray emission rather than a persistent accretion signature. Using the incident high-energy flux, the paper estimates the ionization, heating, and mass-loss rates for CI Tau c and discusses implications for atmospheric evolution and for interpreting the planetary signal in this system.

Abstract

We report results of Chandra X-ray observations of CI Tau, a young magnetically active classical T Tauri star for which previous studies have reported periodic variability attributed to a massive planet in a short-period orbit. CI Tau was clearly detected by Chandra in four separate observations acquired in late 2023. The X-ray emission was steady in the first two observations with a characteristic plasma temperature kT ~ 2 keV (~23 MK) and X-ray luminosity log Lx = 29.74 erg/s. During each of the last two observations obtained two weeks later the count rate increased slowly and the X-ray plasma temperature was much higher but remained nearly steady at kT ~ 4 - 5 keV (~46 - 58 MK) and peak luminosity log Lx = 30.5 erg/s. Such variable X-ray emission in T Tauri stars accompanied by high plasma temperatures is a signature of magnetic activity, consistent with the known presence of a strong magnetic field in CI Tau. We summarize the variable X-ray emission properties of CI Tau within the framework of T Tauri stars of similar mid-K spectral type, identify possible variability mechanisms, and assess the effects of stellar X-ray irradiation on the claimed planet.