Parkes observations for project P1219 semester 2023OCTS_13

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.

磁场可潜在调控恒星形成过程与分子云的演化进程。直接测量星际磁场强度本质上极具挑战，而塞曼分裂（Zeeman splitting）是目前唯一可直接沿视线方向估算磁场强度的方法。尽管已在其他介质中探测到塞曼分裂信号，但截至目前，尚未在宁静态分子云或冷中性介质中针对致密背景源实现塞曼分裂的探测。具有极小立体角与相对较高横向速度的脉冲星，是研究分子云磁场的理想背景源，可为塞曼分裂测量提供清晰的对照信号。目前已在4颗脉冲星的羟基吸收（OH absorption）谱中探测到相关信号，分别为PSR B1849+00、B1641-45、B1718-35与B1749-28。我们计划利用这4颗脉冲星，通过不同观测历元下的羟基吸收塞曼分裂与旋转量（rotation measure）估算，探究分子云内的磁场特性及其变化规律。若能成功探测到相关信号，将为难以测量的分子云磁场研究打开全新窗口，且不受特定解释框架的制约，从而为恒星形成与星际介质的物理机制研究提供新的见解。