Global 3D Simulations of Disc Accretion onto the classical T Tauri Star V2129 Oph

Abstract

The magnetic field of the classical T Tauri star V2129 Oph can be modeled approximately by superposing slightly tilted dipole and octupole moments, with polar magnetic field strengths of 0.35kG and 1.2kG respectively (Donati et al. 2007). Here we construct a numerical model of V2129 Oph incorporating this result and simulate accretion onto the star. Simulations show that the disk is truncated by the dipole component and matter flows towards the star in two funnel streams. Closer to the star, the flow is redirected by the octupolar component, with some of the matter flowing towards the high-latitude poles, and the rest into the octupolar belts. The shape and position of the spots differ from those in a pure dipole case, where crescent-shaped spots are observed at the intermediate latitudes. Simulations show that if the disk is truncated at the distance of 6.2 R_* which is comparable with the co-rotation radius, 6.8 R_*, then the high-latitude polar spots dominate, but the accretion rate obtained from the simulations is about an order of magnitude lower than the observed one. The accretion rate matches the observed one if the disk is disrupted much closer to the star, at 3.4 R_*. However, the octupolar belt spots strongly dominate. Better match has been obtained in experiments with a dipole field twice as strong. The torque on the star from the disk-magnetosphere interaction is small, and the time-scale of spin evolution, 2 x10^7-10^9 years is longer than the 2x10^6 years age of V2129 Oph. The external magnetic flux of the star is strongly influenced by the disk: the field lines connecting the disk and the star inflate and form magnetic towers above and below the disk. The potential (vacuum) approximation is still valid inside the Alfvén (magnetospheric) surface where the magnetic stress dominates over the matter stress.