Quasi periodic oscillations from unstable accretion in CTTSs: an observer's perspective

Three-dimensional (3D) magnetohydrodynamic (MHD) simulations have shown that Classical T Tauri Stars (CTTSs) may accrete in either a stable or an unstable regime relative to the Rayleigh-Taylor instability. The transition between these regimes depends on the ratio between the truncation radius and the corotation radius. In the stable regime, accretion produces a polar hot spot which rotates with the stellar rotation period. In the unstable regime, accretion produces hot spots which rotate with the Keplerian rotation period at the truncation radius. Therefore, unstable accretion sometimes produces quasi-periodic oscillations (QPOs) at periods shorter than the stellar rotation period in the light curves. Recent works have shown how a third scenario might exist. When the truncation radius is very close to the star, unstable accretion becomes ordered, proceeding in one or two tongues of matter. This regime is called Magnetic Boundary Layer (MBL) regime. In this talk I will analyze the photometric behavior of three targets, namely HM Lup, RU Lup and TW Hya, which show signatures of unstable accretion in the light curves obtained with the Transiting Exoplanet Survey Satellite (TESS). For HM Lup, I will discuss how a decrease in the detected period between two different TESS light curves is accompanied by the appearance of ordered QPOs, in agreement with what is observed in the simulations. The spectroscopic data show that this variability is driven by an increase in the accretion rate. For RU Lup, I will show that quasi-periodic oscillations in the TESS light curve are at a period shorter than the stellar rotation period, and how the variability in the detected period is related to the accretion rate variability. In this case, the knowledge of the stellar mass and period allows a direct comparison to the MHD simulations. The ratio of the truncation radius to the corotation radius suggests that RU Lup is accreting through a MBL, with a magnetosphere as small as 1.8 stellar radii. For TW Hya, I will discuss preliminary results on the analysis of the TESS light curves, which also show signatures of unstable accretion.