2016ApJ...826..206J

The ∼2 Myr old classical T Tauri star CI Tau shows periodic variability in its radial velocity (RV) variations measured at infrared (IR) and optical wavelengths. We find that these observations are consistent with a massive planet in a ∼9 day period orbit. These results are based on 71 IR RV measurements of this system obtained over five years, and on 26 optical RV measurements obtained over nine years. CI Tau was also observed photometrically in the optical on 34 nights over ∼one month in 2012. The optical RV data alone are inadequate to identify an orbital period, likely the result of star spot and activity-induced noise for this relatively small data set. The infrared RV measurements reveal significant periodicity at ∼9 days. In addition, the full set of optical and IR RV measurements taken together phase coherently and with equal amplitudes to the ∼9 day period. Periodic RV signals can in principle be produced by cool spots, hotspots, and reflection of the stellar spectrum off the inner disk, in addition to resulting from a planetary companion. We have considered each of these and find the planet hypothesis most consistent with the data. The RV amplitude yields an Msini of ∼8.1 M_Jup; in conjunction with a 1.3 mm continuum emission measurement of the circumstellar disk inclination from the literature, we find a planet mass of ∼11.3 M_Jup, assuming alignment of the planetary orbit with the disk.