Measuring the Spot Variability of T Tauri Stars Using Near-IR Atomic Fe and Molecular OH Lines

Abstract

As part of the Young Exoplanets Spectroscopic Survey (YESS), this study explores the spot variability of 13 T Tauri Stars (TTSs) in the near-infrared $H$ band, using spectra from the Immersion GRating INfrared Spectrometer (IGRINS). By analyzing effective temperature (Teff) sensitive lines of atomic FeI at $\sim$1.56259 $\mu$m and $\sim$1.56362 $\mu$m, and molecular OH at $\sim$1.56310 and $\sim$1.56317 $\mu$m, we develop an empirical equivalent width ratio (EWR) relationship for Teff in the range of 3400–5000 K. This relationship allows for precise relative Teff estimates to within tens of Kelvin and demonstrates compatibility with solar metallicity target models. However, discrepancies between observational data and model predictions limit the extension of the Teff–EWR relationship to a broader parameter space. Our study reveals that both classical and weak-line TTSs can exhibit Teff variations exceeding 150 K over a span of two years. The detection of a quarter-phase delay between the EWR and radial velocity phase curves in TTSs indicates spot-driven signals. A phase delay of 0.06 $\pm$ 0.13 for CI Tau, however, suggests additional dynamics, potentially caused by planetary interaction, inferred from a posited 1:1 commensurability between the rotation period and orbital period. Moreover, a positive correlation between Teff variation amplitude and stellar inclination angle supports the existence of high-latitude spots on TTSs, further enriching our understanding of stellar surface activity in young stars.