Unstable accretion in TW Hya: 3D simulations and comparisons with observations

TL;DR

This study uses 3D MHD simulations to explore the unstable accretion processes in TW Hya, linking magnetic field variations to observed stochastic light curves and quasiperiodic oscillations.

Contribution

It introduces detailed 3D MHD models of TW Hya's accretion, revealing how magnetic tilt and strength influence variability and QPOs, aligning simulations with observations.

Findings

Unstable accretion produces multiple hot spots and stochastic light curves.

Magnetic tilt affects the dominance of stellar or disc-related QPOs.

Longer-period QPOs originate beyond the corotation radius.

Abstract

We investigate the origin of photometric variability in the classical T Tauri star TW Hya by comparing light curves obtained by TESS and ground-based telescopes with light curves created using three-dimensional (3D) magnetohydrodynamic (MHD) simulations. TW Hya is modeled as a rotating star with a dipole magnetic moment, slightly tilted about the rotational axis. We observed that for various model parameters, matter accretes in the unstable regime and produces multiple hot spots on the star's surface, which leads to stochastic-looking light curves similar to the observed ones. Wavelet and Fourier spectra of observed and modeled light curves show multiple quasiperiodic oscillations (QPOs) with quasiperiods from less than 0.1 to 9 days. 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 days) QPOs can be caused by waves forming beyond the corotation radius.