报告题目：(Sub)millimeter disk polarization: a brand new window to study protoplanetary disks

　　报告人：杨海峰 博士

　　报告人简介：Haifeng Yang is currently a C.N. Yang postdoctoral fellow at Institute for Advance Study at Tsinghua University. He got a Ph.D in astronomy in 2018 from University of Virginia and a BSc in Physics in 2011 from Peking University. He is primarily interested in the studies of protoplanetary disks, using polarized thermal emission observations, radiative transfer modeling, and magnetohydrodynamic simulations. He is also interested in the grain alignment theory and the optical properties of dust grains.

　　报告摘要：(Sub)millimeter disk polarization is an exciting new field that is being revolutionized by Atacama Large Millimeter/submillimeter Array (ALMA). Traditionally, (sub)millimeter polarization thermal emission from dusts were believed to trace the magnetic field structures, based on the assumption that grains are aligned with the background magnetic fields. Using polarization to trace magnetic field structure on the disk scale, however, has been a failure. Most systems show uniform polarization patterns, which can be well-explained with the scattering off dust grains themselves, the so-called “self-scattering” or “scattering-induced” mechanism. This opens up a brand new window to study the grain properties in protoplanetary disks (PPDs). Most notably, we have shown that the dust grains have grown to 100 micron in sizes in PPDs. These large dust grains are the building blocks of planetesimals and planets. Aside from probing grain sizes, it is also potentially capable to probe the settling of dust grains, among others.

　　Not all data can be explained with the self-scattering mechanism. The HL Tau Band 3 polarized image, for example, shows an azimuthal pattern which is beyond the scattering-induced mechanism. Aside from scattering, other mechanisms to produce disk polarization all rely on the dust grains being aligned with some underlying fields. Theoretically, the traditional magnetic alignment mechanism is expected to fail due to the disalignment by the frequent random bombardment from the gas particles. The favored grain alignment theory predicts the alignment of dust grains by the radiation flux. This mechanism, however, failed to explain the HL Tau Band 3 data, which can be explained by an old theory proposed by Gold in 1950s. This Gold mechanism is not favored in theory, because it is known to work only under (super) thermal conditions, but the differential motion in disks can hardly achieve thermal speed. The exact origin of complicated polarization patterns is still an open question. If we can settle on their origin, we will be able to harness the full power of ALMA polarization observation to study various properties of PPDs.

　　报告时间：2021年7月12日 (周一) 上午10:00

　　报告地点：5号楼516会议室

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