Wednesday Lunch 4/2: Xuening Bai (CfA)
Transport and Accretion Processes in Protoplanetary Disks: A New Paradigm
Dr. Xuening Bai (Hubble Fellow, CfA)
CAS 500, 12:00 pm, Wednesday April, 2, 2014
The structure and evolution of protoplanetary disks (PPDs) are largely determined by the process of angular momentum transport, and are key to understanding many aspects of planet formation. Due to extremely weak level of ionization, the gas dynamics of PPDs is largely controlled by non-ideal magnetohydrodynamical effects, including Ohmic resistivity, Hall effect and ambipolar diffusion (AD). Most previous studies considered only the effect of Ohmic resistivity under the framework of magnetorotational instability (MRI) driven accretion with dead zones. We show via self-consistent local numerical simulations that the inclusion of ambipolar diffusion (AD) qualitatively changes the conventional picture. In the inner disk around 1 AU, the MRI is suppressed due to the inclusion of AD, and a strong magnetocentrifugal wind can be launched that efficiently drives disk accretion. With a parameter survey, we find that wind-dominated accretion with laminar disk is likely to extend to about 5-10 AU. Beyond this radius, angular momentum transport is likely to proceed through a combination of wind and the MRI, and eventually dominated by the MRI in the outer disk. Ongoing work to include the Hall effect further introduces modest changes to the wind scenario, and also makes the gas dynamics depend on the polarity of the external magnetic field. Our simulation results provide key ingredients for a new paradigm on the transport and accretion processes in PPDs.