Confronting FU Orionis Outbursts and Pre/transitional Disks: Protostellar/ protoplanetary Disk Structure and Evolution.

The mechanisms and efficiencies of mass transport in accretion disks can be best constrained by studying their time-dependent behavior. For the first part of my thesis work, I studied outbursts of rapid accretion in protostellar disks. This was motivated by observations of the FU Orionis objects, which are young stellar objects with a sudden increase of their brightness. I first constructed disk radiative transfer models to compare with Spitzer observations, and found that the outburst disk region extends to scales of an AU or more, with the inferred limits on the viscosity parameter alpha~0.02-0.2. I further analyzed the multi-wavelength high resolution spectra to show the outbursting disk follows Keplerian rotation. I next studied the thermal structure of the disk which leads us to propose that the outburst is due to the thermal activation of the magnetorotational instability (MRI) at ~AU scales by the gravitational instability (GI). We carried out both analytic studies and 2-Dimensional radiationhydrodyanmic simulations to study the outbursts and constrain the activation of the MRI. Then I extended this work by constructing simplified 1-dimensional time-dependent simulations with infall from the protostellar envelope to study protostellar disk formation and long term evolution. I found that the outbursting behavior of the protostellar disks during the infall phase alleviates the so called "luminosity problem", and the layered accretion at the later phase leads to a massive dead zone where planet formation is favored. In the second part of my thesis work, I studied protoplanetary disks with gaps and holes-so called "pretransitional and transitional disks",- to study planet formation in young disks. I have studied gap opening by dynamically-interacting multiple giant planets with two-dimensional hydrodynamic simulations. I found that even with as many as four giant planets, additional substantial dust depletion (e.g. growth) is required to explain these gaps and holes, which sheds light on the early planet formation environment.Ph.D.Astronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/86332/1/zhuzh_1.pd