Wednesday 10 Jun 2015: Seminar: Formation of Close-in Super-Earths via an MRI-Induced Pressure Barrier
Dr Subhanjoy Mohanty - Imperial College London
Physics, 4th floor 14:00-15:00
Multiple super-Earths packed in very close to the central star appear to be a major planet formation pathway around low-mass stars, comprising ~50% of planetary systems around these stars. Neither planet-planet scattering, nor formation at larger radii followed by inward migration, appears capable of explaining most of these systems. Instead, an attractive scenario (proposed by Chatterjee & Tan) is the inward migration of solid particles from larger disk radii due to gas drag, followed by the trapping of these particles at a pressure barrier close to the disk inner edge; super-Earths can then form efficiently within the trap. The pressure barrier in turn is caused by a presumed sharp drop in disk viscosity as one goes from the thermally ionised innermost disk regions (where the magneto-rotational instability -- MRI -- can operate efficiently) to larger disk radii where the ionisation drops drastically (thereby shutting off the MRI). We present initial calculations of the MRI-induced disk viscosity, accounting for realistic non-ideal MHD effects in the disk, for various accretion rates. The resultant properties of the pressure barrier are discussed, as well as the attendant implications for the viability of this mechanism for planet formation. We also present a novel mechanism for explaining why ~50% of low-mass stars do NOT form such planets (e.g., our own solar system).