The magnetospheric gap and the accumulation of giant planets close to a star

The bunching of giant planets at a distance of several stellar radii may be explained by the disruption of the inner part of a disk by the magnetosphere of a star during the T Tauri stage of evolution. The rotating magnetic field of the star gives rise to a low-density magnetospheric gap where stellar migration is strongly suppressed. We performed full three-dimensional (3D) magnetohydrodynamic simulations of the disk-magnetosphere interaction and examined conditions for which the magnetospheric gap is empty, by changing the misalignment angle between the magnetic and rotational axes of the star, theta, and by lowering the adiabatic index g, which is a mock-up of the effect of heat conductivity and cooling. Our simulations show that for a wide range of plausible conditions, the gap is essentially empty. However, in the case of large misalignment angles theta, part of the funnel stream is located in the equatorial plane, and the gap is not empty. Furthermore, if the adiabatic index is small (gamma similar to 1.1), and the rotational and magnetic axes are almost aligned, then matter penetrates through the magneg tosphere due to 3D instabilities, forming high-density equatorial funnels. For these two limits, there is appreciable matter density in the equatorial plane of the disk so that a planet may migrate into the star.