Accretion to stars with non-dipole magnetic fields

Abstract

ABSTRACT Disc accretion to a rotating star with a non-dipole magnetic field is investigatedfor the first time in full three-dimensional (3D) magnetohydrodynamic (MHD) simula-tions. We investigated the cases of (1) pure dipole, (2) pure quadrupole, and (3) dipoleplus quadrupole fields. The quadrupole magnetic moment D is taken to be parallelto the dipole magnetic moment µ, and both are inclined relative to the spin axis ofthe star Ω at an angle Θ. Simulations have shown that in each case the structure ofthe funnel streams and associated hot spots on the surface of the star have specificfeatures connected with the magnetic field configuration. In the pure dipole case mat-ter accretes in two funnel streams which form two arch-like spots near the magneticpoles. In the case of a pure quadrupole field, most of the matter flows through thequadrupole “belt” forming a ring-shaped hot region on the magnetic equator. In thecase of a dipole plus quadrupole field, magnetic flux in the northern magnetic hemi-sphere is larger than that in the southern, and the quadrupole belt and the ring aredisplaced to the south. The stronger the quadrupole, the closer the ring is to the mag-netic equator. At sufficiently large Θ, matter also flows to the south pole, forming a hotspot near the pole. The light curves have a variety of different features which makesit difficult to derive the magnetic field configuration from the light curves. There arespecific features which are different in cases of dipole and quadrupole dominated mag-netic field: (1) Angular momentum flow between the star and disc is more efficient inthe case of the dipole field; (2) Hot spots are hotter and brighter in case of the dipolefield because the matter accelerates over a longer distance compared with the flow ina quadrupole case.Key words: accretion, accretion discs - magnetic fields - MHD - stars: magneticfields.