Global structure and evolution of protoplanetary disks (PPDs) are governed by physical processes that drive disk angular momentum transport 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 the 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.

Bio: Dr. Xuening Bai obtained his B.S. in mathematics and physics from Tsinghua University in 2007, and his PhD in astrophysics from Princeton University in 2012. He then moved to Institute for Theory and Computation (ITC) at Harvard-Smithsonian Center for Astrophysics with a joint Hubble Fellow (2012-2015) and ITC fellow (2015-present). He was also appointed as a lecturer at Harvard University in 2015-2016. Dr. Bai is a theoretical and computational astrophysicist. He is primarily interested in planet formation, particularly on the dynamics of gas and dust in protoplanetary disks. He is also interested in several aspects of high-energy astrophysics, including accretion disks, pulsar physics, and cosmic-ray acceleration and transport.