Measuring interstellar magnetic fields is extremely important for understanding their role in different evolutionary stages of interstellar clouds and of star formation. However, detecting the weak field is observationally challenging. We present measurements of the Zeeman effect in the 1665 and 1667~MHz (18~cm) lines of the hydroxyl radical (OH) lines toward the dense photodissociation region (PDR) associated with the compact H{\sc ii} region DR~21~(Main). From the OH 18~cm absorption, observed with the Karl G. Jansky Very Large Array, we find that the line of sight magnetic field in this region is $\sim 0.13$~mG. The same transitions in maser emission toward the neighboring DR~21(OH) and W~75S-FR1 regions also exhibit the Zeeman splitting. Along with the OH data, we use [C{\sc ii}] 158 $\mu$m line and hydrogen radio recombination line data to constrain the physical conditions and the kinematics of the region. We find the OH column density to be $\sim 3.6\times10^{16}(T_{\rm ex}/25~{\rm K})~{\rm cm}^{-2}$, and that the 1665 and 1667 MHz absorption lines are originating from the gas where OH and C$^+$ are co-existing in the PDR. Under reasonable assumptions, we find the measured magnetic field strength for the PDR to be lower than the value expected from the commonly discussed density--magnetic field relation while the field strength values estimated from the maser emission are roughly consistent with the same. Finally, we compare the magnetic field energy density with the overall energetics of DR~21's PDR and find that, in its current evolutionary stage, the magnetic field is not dynamically important.

中文翻译：

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DR 21 致密光解区的磁场

测量星际磁场对于理解它们在星际云和恒星形成的不同演化阶段中的作用极其重要。然而，检测弱场在观察上具有挑战性。我们在 1665 和 1667~MHz (18~cm) 线的羟自由基 (OH) 线中测量了塞曼效应，该线朝向与致密 H{\sc ii} 区域 DR~21 相关的致密光解离区 (PDR) ～（主要）。从使用 Karl G. Jansky 甚大阵列观察到的 OH 18~cm 吸收，我们发现该区域的视线磁场为 $\sim 0.13 $~mG。向相邻的 DR~21(OH) 和 W~75S-FR1 区域的脉泽发射的相同转变也表现出塞曼分裂。与 OH 数据一起，我们使用 [C{\sc ii}] 158 $\mu$m 线和氢无线电复合线数据来约束该区域的物理条件和运动学。我们发现 OH 柱密度为 $\sim 3.6\times10^{16}(T_{\rm ex}/25~{\rm K})~{\rm cm}^{-2}$，并且1665 和 1667 MHz 吸收谱线源自于 PDR 中 OH 和 C$^+$ 共存的气体。在合理的假设下，我们发现 PDR 的测量磁场强度低于通常讨论的密度 - 磁场关系的预期值，而根据脉泽发射估计的场强值大致一致。最后，我们将磁场能量密度与 DR~21 的 PDR 的整体能量进行比较，发现在其当前的演化阶段，磁场在动态上并不重要。