UCLA

We present a physically motivated model for the manner in which a stellar magnetic field sculpts the inner edge of a protoplanetary disk, and examine the consequence for the migration and stopping of sub-Neptune and super-Earth planets.This model incorporates a transition zone exterior to the inner truncation of the disk, where the surface density profile is modified by the diffusion of the stellar magnetic field into the disk. This modification results in a migration trap at the outer edge of the transition zone. We first performed simulations of single planet migration, considering a range of stellar magnetic field strengths and magnetic diffusion profiles. Our simulations show a tight relationship between the final locations of planets and the total magnetic budget available for the disk from their host star. We found that a stellar magnetic field between 67 to 180G and a power-law index between 3 and 2.75 can reasonably reproduce the location at which the observed occurrence rate of close-in Super-Earth and Sub-Neptune populations changes slope. We then performed simulations of double planet migration for a dipole stellar magnetic field of 300G. Our simulations show a tight relationship between the planets' mass ratio and their next neighbor orbital period ratio. We also found that our model is capable of broadly reproducing some features seen in Kepler's multi-planet systems. Such features include the 3:2 resonance peak on the next neighbor period ratio diagram, a collection of systems in non-resonance configurations, a drop off in planet occurrence rate below {\color{red} \bf 9} days and a plateau in planet occurrence beyond 9 days.