Three-dimensional Simulations of Accretion to Stars with Complex Magnetic Fields

Abstract

We investigate disk accretion onto rotating stars with different complex magnetic fields, including superpositions of misaligned dipole and quadrupole, off‐centered dipoles and so on. The full three‐dimensional magnetohydrodynamic simulations show that when the quadrupole component is comparable to the dipole component, the magnetic field has a complex structure with three major magnetic poles on the surface of the star and three sets of loops of field lines connecting them. A significant amount of matter flows to the quadrupole “belt,” forming a ring‐like hot spot on the star. If the maximum strength of the magnetic field on the star is fixed, then we observe that the mass accretion rate, the torque on the star, and the area covered by hot spots are several times smaller in quadrupole dominant cases than in the pure dipole case. The influence of the quadrupole component on the shape of the hot spots becomes noticeable when the ratio of quadruple to dipole strengths of the field Bq/Bd≳0.5, and becomes dom...