Dynamics of magnetic flux tubes in accretion discs of T Tauri stars

Abstract

Dynamics of slender magnetic flux tubes (MFTs) in the accretion discs of T Tauri stars is investigated. We perform simulations taking into account buoyant, aerodynamic, and turbulent drag forces, radiative heat exchange between MFT and ambient gas, and magnetic field of the disc. The equations of MFT dynamics are solved using Runge-Kutta method of the fourth order. The simulations show that there are two regimes of MFT motion in absence of external magnetic field. In the region r < 0.2 au, the MFTs of radii 0.05 ≤ a0 ≤ 0.16, H (H is the scale height of the disc) with initial plasma beta of 1 experience thermal oscillations above the disc. The oscillations decay over some time, and MFTs continue upward motion afterwards. Thinner or thicker MFTs do not oscillate. MFT velocity increases with initial radius and magnetic field strength. MFTs rise periodically with velocities up to 5-15 km s-1 and periods of 0.5-10 yr determined by the toroidal magnetic field generation time. Approximately 20 per cent of disc mass and magnetic flux can escape to disc atmosphere via the magnetic buoyancy over characteristic time of disc evolution. MFTs dispersal forms expanding magnetized corona of the disc. External magnetic field causes MFT oscillations near the disc surface. These magnetic oscillations have periods from several days to 1-3 months at r < 0.6 au. The magnetic oscillations decay over few periods. We simulate MFT dynamics in accretion discs in the Chameleon I cluster. The simulations demonstrate that MFT oscillations can produce observed IR-variability of T Tauri stars. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.