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Speaker:

Zhi-Yun Li (University of Virginia)

Topic:

Disk Polarization as a Probe of Grain Properties and Kinematics in Protoplanetary Disks

Abstract:

Spatially resolved disk continuum polarization at (sub)millimeter and centimeter wavelengths was first detected using the SMA, CARMA, and VLA, and has since been revolutionized by ALMA. Initially, the observed disk polarization was attributed to direct emission from magnetically aligned grains. If confirmed, this would provide the long-sought evidence of magnetic fields expected to play a crucial role in disk dynamics and evolution. However, alternative explanations have emerged, including scattering by large grains and grain alignment with respect to the direction of radiative flux. In this talk, I will first explore in detail the scattering-induced disk polarization, emphasizing the effects of disk inclination relative to the line of sight, optical depth, and the potential of polarization observations to probe grain growth and settling in protoplanetary disks. I will then present evidence supporting the co-existence of direct polarized thermal emission from toroidally aligned prolate (TOP) grains alongside scattering by the same grains. Finally, I will discuss a plausible scenario for the inferred toroidal grain alignment, driven by gas aerodynamic drag on grains in Keplerian motion.