LPL Colloquium - Dr. Xuening Bai

ITC Fellow and Lecturer
Harvard College Observatory

Global structure and evolution of protoplanetary disks (PPDs) are governed by physical processes that drive angular momentum transport (which leads to mass accretion onto the central protostar) and mass loss, and they are the key to understanding almost every aspect of planet formation. Conventionally, disk evolution is thought to be viscously driven, as a result of turbulence generated by the magneto-rotational instability (MRI). However, the weakly ionized nature of PPDs introduces non-ideal magneto-hydrodynamic (MHD) effects that substantially modify the coupling between gas and magnetic fields in complex ways. I first show that these effects lead to suppression or damping of the MRI in most regions of PPDs, and instead, global disk evolution is driven primarily by a magnetized disk wind. Next, I describe a semi-analytical model to characterize the wind properties, and show that they are determined by the amount of external magnetic flux threading the disks, as well as disk thermal structure. For typical PPDs, wind mass loss rate is likely comparable to accretion rate, and mass loss is most significant in the outer disk (>10 AU). This will substantially enhance dust-to-gas mass ratio in PPDs and promote planet formation. Finally, I describe the first global simulations of PPDs that aim to incorporate the most realistic disk microphysics, and fully accommodate disk wind in the computational domain. In the near future, these simulations will allow us to explore global disk structure and long-term evolution from first principles, and to reassess the theory of planet formation based on the new paradigm of PPD gas dynamics.