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ABSTRACT We investigate the origin of photometric variability in the classical T Tauri star TW Hya by comparing light curves obtained by Transiting Exoplanet Survey Satellite (TESS) and ground-based telescopes with light curves created using three-dimensional (3D) magnetohydrodynamic (MHD) simulations. TW Hya is modelled as a rotating star with a dipole magnetic moment, which is slightly tilted about the rotational axis. We observed that for various model parameters, matter accretes in the unstable regime and produces multiple hotspots on the star’s surface, which leads to stochastic-looking light curves similar to the observed ones. Wavelet and Fourier spectra of observed and modelled light curves show multiple quasi-periodic oscillations (QPOs) with quasi-periods from less than 0.1 to 9 d. Models show that variation in the strength and tilt of the dipole magnetosphere leads to different periodograms, where the period of the star may dominate or be hidden. The amplitude of QPOs associated with the stellar period can be smaller than that of other QPOs if the tilt of the dipole magnetosphere is small and when the unstable regime is stronger. In models with small magnetospheres, the short-period QPOs associated with rotation of the inner disc dominate and can be mistaken for a stellar period. We show that longer period (5–9 d) QPOs can be caused by waves forming beyond the corotation radius. 

Abstract Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto stars via funnel flows, which follow stellar magnetic field lines and shock on the stellar surfaces 1–3 , leaving hot spots with density gradients 4–6 . Previous work has provided observational evidence of varying density in hot spots 7 , but these observations were not sensitive to the radial density distribution. Attempts have been made to measure this distribution using X-ray observations 8–10 ; however, X-ray emission traces only a fraction of the hot spot 11,12 and also coronal emission 13,14 . Here we report periodic ultraviolet and optical light curves of the accreting star GM Aurigae, which have a time lag of about one day between their peaks. The periodicity arises because the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of a magnetospheric accretion model, this finding indicates the presence of a radial density gradient in a hot spot on the stellar surface, because regions of the hot spot with different densities have different temperatures and therefore emit radiation at different wavelengths.