Global 3D simulations of disc accretion on to the classical T Tauri star BP Tauri

Abstract

The magnetic field of the classical T Tauri star BP Tau can be approximated\nas a superposition of dipole and octupole moments with respective strengths of\nthe polar magnetic fields of 1.2 kG and 1.6 kG (Donati et al. 2008). We adopt\nthe measured properties of BP Tau and model the disc accretion onto the star.\nWe observed in simulations that the disc is disrupted by the dipole component\nand matter flows towards the star in two funnel streams which form two\naccretion spots below the dipole magnetic poles. The octupolar component\nbecomes dynamically important very close to the star and it redirects the\nmatter flow to higher latitudes. The spots are meridionally elongated and are\nlocated at higher latitudes, compared with the pure dipole case, where\ncrescent-shaped, latitudinally elongated spots form at lower latitudes. The\nposition and shape of the spots are in good agreement with observations. The\ndisk-magnetosphere interaction leads to the inflation of the field lines and to\nthe formation of magnetic towers above and below the disk. The magnetic field\nof BP Tau is close to the potential only near the star, inside the\nmagnetospheric surface, where magnetic stress dominates over the matter stress.\nA series of simulation runs were performed for different accretion rates. They\nshow that an accretion rate is lower than obtained in many observations, unless\nthe disc is truncated close to the star. The torque acting on the star is about\nan order of magnitude lower than that which is required for the rotational\nequilibrium. We suggest that a star could lose most of its angular momentum at\nearlier stages of its evolution.\n