Magnetospheric Gap and Accumulation of Giant Planets Close to the Star

Abstract

The bunching of giant planets at a distance of several stellar radii may be explained by the disruption of the inner part of the disk by the magnetosphere of the 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 3D magnetohydrodynamic simulations of the disk-magnetosphere interaction and examined conditions for which the magnetospheric gap is "empty", by changing the misalignment angle between magnetic and rotational axes of the star, Theta, and by lowering the adiabatic index gamma, which mocks up 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=1.1) and the rotational and magnetic axes are almost aligned, then matter penetrates through the magnetosphere 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.