MHD simulations on accretion in T Tauri stars triggered by coronal flares
T Tauri stars are young (typically younger than 10 million years) stars surrounded by a protoplanetary disk, which is a disk of gas and dust orbiting around the star and that eventually can evolve in planetary systems. Despite these stars have already accreted most of their mass, they can still accrete some mass from their protoplanetary disks. The accretion process is driven by the stellar magnetic field. In proximity of the central star (e.g. closed than some 0.1 AU, astronomical units, which is the average Earth-Sun distance), in fact, the stellar magnetic field can be intense enough to produce a magnetic pressure that overcome the gas pressure. In these cases, the disk gas is funneled by the magnetic field and it accretes onto the central star at typical free-fall velocities of some hundreds km/sec.
T Tauri stars are also characterized by an intense magnetic activity, producing stellar flares that are more frequent and energetic than those occurring in main sequence stars such as the Sun. As demontrated by several studies, during flares in T Tauri stars the plasma can be heated up to 100 million degrees, emitting energetic X-ray radiation, in loops that can be long enough to connect the central star with its protoplanetary disk.
The research led by S. Colombo (INAF – Astronomical Observatory of Palermo and University of Palermo, now at the Observatoire de Paris and Sorbonne University) studies the effects of bright X-ray flares in T Tauri stars on their protoplanetary disk. Following the analysis in Orlando et al. 2011, the authors have developed a 3D magnetohydrodynamic model of a T Tauri star with a protoplanetary disk, accounting for the stellar gravity, disk viscosity, thermal conduction, radiative loss from the optically thin plasma and the stellar flare by a parametrized heating function. The model is applied to disks with different density and flares of different intensity, in order to explore the space parameters. The study shows that flares can form loops which connect the star and the disk, that can significantly perturb the stability of the disk. One of the most important consequences is the triggering of pressure waves that travel along the disk, and that can contribute to the formation of accretion columns onto the central star, with a typical accretion rate of 10-10, 10-9 solar masses per year (which are typical values observed in T Tauri stars). These accretion funnels can interact each other, even merge in larger channels, resulting in an unstable accretion process. The simulations are described in the paper “New view of the corona of classical T Tauri stars: Effects of flaring activity in circumstellar disks“, published by Astronomy & Astrophysics. The astronomers S. Orlando and R. Bonito of INAF – Astronomical Observatory of Palermo, and C. Argiroffi, G. Peres, and F. Reale of the University Palermo are coauthors of the study.
The figure (link) shows several snapshots taken during the evolution of the system. In each snapshot, the upper panel shows the section of the star-disk system, which the upper panels the view from the poles. The density is marked with a blue scale, while temperature in red-yellow. The green lines show the magnetic field lines. The white arrow shows the rotation of the disk.
by Mario Giuseppe Guarcello ( follow mguarce)