Orkan Umurhan (SETI/NASA Ames)
622 Space Sciences
Abtract: The early epochs of protoplanetary disks, during which the first planetesimals form, are characterized as being gas-rich whose bulk interiors are magnetically inactive. Up until recently, these Ohmic zones were thought to be dynamically inactive and, thus, have been described as "Dead Zones". Recent theoretical and numerical advances have shown that, in fact, these zones are not dead afterall -- they support 3 kinds of turbulence generating instabilities. In this talk I will give a tour of the character and quality of the Convective Overstability, the Vertical Shear Instability, and the Zombie Vortex Instability, and describe how they transition into turbulence. All three mechanisms appear to generate disk alphas in the range 5e-5 - 1e-3, which is consistent with upper limits implied by observations. All three mechanisms operate under mutually exclusive thermal conditions which correspond to similarly exclusive regions of protoplanetary disks. In the second part of this tour, I will discuss the implications that the presence of these instabilities has upon planetesimal formation, namely, its consequences for the dust accumulating Streaming Instability process. We find that the presence of low-levels of turbulence can throttle the ability of planetesimals to grow beyond centimeters sized grains (Stokes numbers less than 0.01).