Parkes observations for project P1219 semester 2023OCTS_18

The magnetic field potentially regulates the process of star formation and the evolution of molecular clouds. It is inherently difficult to measure interstellar magnetic field strengths, with the measurement of Zeeman splitting a unique method to estimate the magnetic field strength along the line of sight directly. Despite the detection of Zeeman splitting in other mediums, there are as yet no Zeeman detections against compact background sources in quiescent molecular clouds or the cold neutral medium. Pulsars with extremely small solid angles and relatively high transverse velocities are ideal background sources to study the magnetic field in molecular clouds, providing a distinct signal to measure splitting against. There are four pulsars with OH absorption detections, namely PSR B1849+00, B1641-45, B1718-35, and B1749-28. We propose to utilize these four pulsars to explore the properties of the magnetic field and its variations within molecular clouds through both the Zeeman splitting of OH absorption and rotation measure estimations, between epochs. If a detection is confirmed, it will open a new window on the hard-to-measure magnetic fields in molecular clouds, independent of interpretation, thus shedding light on the physics of star formation and the interstellar medium.

磁场可对恒星形成过程与分子云的演化起到调控作用。星际磁场强度的测量本身具有本质难度，而塞曼分裂（Zeeman splitting）测量法是唯一可直接沿视线方向估算磁场强度的手段。尽管已在其他介质中探测到塞曼分裂信号，但截至目前，尚无在宁静分子云或冷中性介质中针对致密背景源的塞曼分裂探测记录。具有极小立体角与较高横向速度的脉冲星（pulsar），是研究分子云磁场的理想背景源，可提供清晰的分裂测量参考信号。目前已探测到四颗存在羟基（OH）吸收信号的脉冲星，分别为PSR B1849+00、B1641-45、B1718-35与B1749-28。本研究拟利用这四颗脉冲星，通过不同观测时段下的羟基吸收塞曼分裂与旋转测量（rotation measure）估算，探究分子云内磁场的特性及其变化规律。若该探测得以证实，将为难以直接测量的分子云磁场研究打开全新的、无需依赖额外诠释假设的窗口，从而为恒星形成与星际介质的物理机制研究提供新的指引。