题目：Origins of (sub)millimeter disk polarization 

　时间：2019年7月5日（星期五）上午10:00—11:00 

　地点：天文大厦1416会议室 

　报告人：Dr. Haifeng Yang (杨海峰), Tsinghua University 

　摘要： 

　　Polarized (sub)millimeter dust thermal emission has been established as an important tool to probe magnetic fields. Its application to the protoplanetary disk, however, hasn't been successful. The first resolved polarization map in a classical T Tauri system, HL Tau, yields a uniform magnetic fields, which is a rather unphysical configuration. We were then motivated to search for alternatives and find that the scattering of dust thermal emission by dust grains themselves can naturally produce the observed uniform pattern. I will first review how scattering-induced polarization works and why it has the potential to study the grain growth, as well as to probe dust settling, which is hard to do otherwise. I will then focus on the best studied system, the HL Tau system. This shows a transition from scattering to a complete different picture as we move from ALMA Band 7 (0.87 mm) to ALMA Band 3 (3 mm). The azimuthal pattern at Band 3 was initially proposed to come from a third mechanism, radiative alignment, which is different from both scattering and magnetic alignment. I will argue that this explanation fails to explain some of the important features and cannot be the whole story. At the end, I will discuss the growing tension between scattering-induced polarization and beta-index method in probing grain sizes. 

　　Haifeng Yang (杨海峰) got PhD in astronomy from University of Virginia in 2018, under the supervision of Prof. Zhi-Yun Li. He was an undergraduate student in Peking University with a physics major. Since 2018 fall, he worked at the institute for advanced study, Tsinghua University (IASTU) as a C. N. Yang Junior Fellow, in collaboration with Xuening Bai. His research focused on resolved polarization observations from protoplanetary disks, and related physics, such as grain growth, dust settling, grain alignment, etc. He is also interested in magnetohydrodynamics (MHD) simulations of protoplanetary disks and multidisciplinary research.