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Measuring interstellar magnetic fields is extremely important forunderstanding their role in different evolutionary stages of interstellarclouds and of star formation. However, detecting the weak field isobservationally challenging. We present measurements of the Zeeman effect inthe 1665 and 1667~MHz (18~cm) lines of the hydroxyl radical (OH) lines towardthe 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 thisregion is $\sim 0.13$~mG. The same transitions in maser emission toward theneighboring 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 radiorecombination line data to constrain the physical conditions and the kinematicsof the region. We find the OH column density to be $\sim3.6\times10^{16}(T_{\rm ex}/25~{\rm K})~{\rm cm}^{-2}$, and that the 1665 and1667 MHz absorption lines are originating from the gas where OH and C$^+$ areco-existing in the PDR. Under reasonable assumptions, we find the measuredmagnetic field strength for the PDR to be lower than the value expected fromthe commonly discussed density--magnetic field relation while the fieldstrength values estimated from the maser emission are roughly consistent withthe same. Finally, we compare the magnetic field energy density with theoverall energetics of DR~21's PDR and find that, in its current evolutionarystage, the magnetic field is not dynamically important.

The magnetic field in the dense photodissociation region of DR 21