Modulated Accretion in the T Tauri Star RY Tau-a Stable MHD Propeller or a Planet at 0.2 au?

Abstract

Planets are thought to form at the early stage of stellar evolution when mass accretion is still ongoing. RY Tau is a T Tauri type star at the age of a few Myr, with an accretion disc seen at high inclination, so that the line of sight crosses both the wind and accretion gas flows. In a long series of spectroscopic monitoring of the star over the period 2013-2020, we detected variations in H$\, {\alpha }$ and Na i D absorptions at radial velocities of infall (accretion) and outflow (wind) with a period of about 22 d. The absorptions in the infalling and outflowing gas streams vary in antiphase: an increase of infall is accompanied by a decrease of outflow, and vice versa. These 'flip-flop' oscillations retain phase over several years of observations. We suggest that this may result from the magnetohydrodynamics processes at the disc-magnetosphere boundary in the propeller mode. Another possibility is that a massive planet is modulating some processes in the disc and is providing the observed effects. The period, if Keplerian, corresponds to a distance of 0.2 au, which is close to the dust sublimation radius in this star. The presence of the putative planet can be confirmed by radial velocity measurements: the expected amplitude is ≥90 m s-1 if the planet mass is ≥2 MJ. © 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.