11/19 Lunch Talk: Protoplanetary Disks
Spatial Variation of Dust Evolution in Protoplanetary Disks
Dr. Melissa McClure
University of Michigan
November 19, 2013
Within circumstellar disks, the properties of the dust dictate the structure of the disk, and the variation in these properties as a function of vertical height and radius can be used to probe dynamical processes such as settling, turbulence, and radial transport. These processes affect the ability to form a proto-planet at a given disk location, while the variety of dust species at each location impacts the final composition of planets formed there. By comparing data from spectroscopic programs using Magellan, Spitzer, and Herschel with physical models of irradiated accretion disks, I aim to characterize the spatial variations in mineralogy, grain size, and dust/gas mass ratio within circumstellar disks and identify how early they occur.
My work thus far finds that even in young star-forming regions (like Ophiuchus at ~ 0.8-1 Myr), disks show signs of significant dust processing and settling. However, grains in the innermost ~0.2 AU may be prevented from growing beyond several microns in disks with high mass accretion, likely a sign of turbulent fragmentation due to viscous dissipation through the midplane. Based on the large solid angle required to fit these disks’ emission excess over their stellar photospheres and comparison against models with a variety of dust opacities, the midplane dust in the highly accreting systems may be rich in FeO while the upper layers are depleted of it, suggesting a vertical gradient in the gas phase oxygen abundance. In the outer regions of the disk, oxygen-rich ices appear to be sequestered in the millimeter sized grains in the midplane due to settling; in less settled systems where we do see ice emission, it appears at radii consistent with a proto-Kuiper belt, sometimes in conjunction with flash-heated silicates. Further work to examine variation in grain size distributions at intermediate radii with updated disk models and spatially resolved data is underway.