Global 3D simulations of disc accretion on to the classical T Tauri star V2129 Oph

Author： Romanova M. M. Long M. Lamb F. K. Kulkarni A. K. Donati J.-F.

ISSN： 0035-8711

Source： Monthly Notices of the Royal Astronomical Society, Vol.411, Iss.2, 2011-02, pp. : 915-928

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Abstract

ABSTRACTThe magnetic field of the classical T Tauri star V2129 Oph can be modelled approximately by superposing slightly tilted dipole and octupole moments, with polar magnetic field strengths of 0.35 and 1.2 kG, respectively, as observed by Donati et al. Here we construct a numerical model of V2129 Oph incorporating this result and simulate accretion on to the star using a three-dimensional magnetohydrodynamic code. Simulations show that the disc 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 disc is truncated at the distance of r≈ 6.2R_* which is comparable with the corotation radius, r_cor≈ 6.8 R_* , then the high-latitude polar spot dominates, but the accretion rate obtained from the simulations (and from the accompanying theoretical calculations) is about an order of magnitude lower than the observed one. The accretion rate matches the observed one if the disc is disrupted much closer to the star, at 3.4R_* . However, in that case the octupolar belt spots strongly dominate. In the intermediate case of r≈ 4.3R_* , the polar spots are sufficiently bright, and the accretion rate is within the error bar of the observed accretion rate, and this model can explain the observations. However, an even better match has been obtained in experiments with a dipole field twice as strong compared with one suggested by Donati et al.The torque on the star from the disc–magnetosphere interaction is small, and the time-scale of spin evolution, 2 × 10⁷–6 × 10⁸ yr is longer than the 2 × 10⁶ yr age of V2129 Oph. This means that V2129 Oph probably lost most of its angular momentum in the early stages of its evolution, possibly, during the stage when it was fully convective, and had a stronger magnetic field. The propeller mechanism could also be responsible for the rapid spin-down.The external magnetic flux of the star is strongly influenced by the disc: the field lines connecting the disc and the star inflate and form magnetic towers above and below the disc. The potential (vacuum) approximation is still valid inside the Alfvén (magnetospheric) surface where the magnetic stress dominates over the matter stress.