Mottled protoplanetary disk ionization by magnetically-channeled T Tauri star energetic particles

Abstract

The evolution of protoplanetary disks is believed to be driven largely by angular momentum transport resulting from magnetized disk winds and turbulent viscosity. The ionization of the disk that is essential for these processes has been thought due to host star coronal X-rays but could also arise from energetic particles produced by coronal flares or by travelling shock waves and advected by the stellar wind. We have performed test-particle numerical simulations of energetic protons propagating into a realistic T~Tauri stellar wind, including a superposed small-scale magnetostatic turbulence. The isotropic (Kolmogorov power spectrum) turbulent component is synthesised along the individual particle trajectories. We have investigated the energy range $[0.1 - 10]$ GeV, consistent with expectations from {\it Chandra} X-ray observations of large flares on T~Tauri stars and with recent indications by the {\it Herschel} Space Observatory of a significant contribution of energetic particles to the disk ionization of young stars. In contrast with a previous theoretical study finding dominance of energetic particles over X-ray in the ionization throughout the disk, we find that the disk ionization is likely dominated by X-rays over much of its area except within narrow regions where particles are channeled onto the disk by the strongly-tangled and turbulent magnetic field. The radial thickness of such regions is $\sim 5$ stellar radii close to the star and broadens with increasing radial distance. This likely continues out to large distances from the star ($10$ AU or greater) where particles can be copiously advected and diffused by the turbulent wind.