分子云磁场与尘埃导致的偏振

Magnetic Fields in Molecular Clouds and Polarization from Dust

磁场对分子云及其中的恒星的形成和演化起到重要的作用.分子云磁场的探测方法主要是谱线塞曼效应、尘埃热辐射的偏振,以及谱线的线偏振观测.利用谱线的塞曼效应可以直接测量视线方向的磁场强度.尘埃热辐射偏振可以有效地示踪磁场方向在天球上的分布.分子云内部的磁场会受到不同物理过程的影响.高分辨率观测可以研究磁场扰动的细节,低分辨率观测可以得到分子云甚至银河系大尺度磁场的宏观信息.只有多波段的观测才能全面地认识分子云磁场与各种物理过程的联系.该文对分子云尘埃热辐射偏振的观测情况做了调研,总结了分子云大尺度磁场的研究现状和发展前景.

Molecular clouds are the birthplace of stars, How does a molecular cloud collapse to form stars？ Quasi-static model shows that magnetic field is the main force to balance gravitational force, while the turbulence model emphasizes that turbulence plays a more important role than magnetic field. It is not clear how important the magnetic field is during the star formation process. Measuring magnetic field of molecular clouds is very crucial to many unanswered questions. There are three methods to measure the magnetic field of molecular clouds： Zeeman split- ting, dust polarization and spectral linear polarization. Magnetic field strength in the clouds can be measured with the first method. Polarization of dust thermal emission is widely used to tracethe field orientations in clouds. Observations at different wavelengths probe different regions of clouds, from a whole cold cloud to the hot core. Magnetic fields are always strongly disturbed near the core because of drastic physical processes. Low resolution observations can reveal the large scale magnetic field structures of molecular clouds. Though magnetic fields inside clouds have been disordered, the large scale magnetic field structures may still be preserved from the fields frozen in interstellar medium, which are expected to be predominately parallel to the Galactic plane. We summarized the low resolution polarized observations of dust thermal emissions, and collected all the results of observed thermal polarization from dust grains. The mean polarization angles of those clouds were calculated and displayed according to the measured scale. Only a few clouds have low resolution observations done, which show the magnetic field aligned to the Galactic plane. Much more powerful telescopes in mm （sub-mm） wavebands have been proposed to do observations with much higher resolutions and sensitivities. We noticed that some lower resolution （〉 1＇） observations in the same wavebands covering the whole sky in future will show much clearer images of large scale magnetic field structures of molecular clouds.