The magnetic field in the dense photodissociation region of DR 21

TL;DR

This study measures the magnetic field in the dense PDR of DR 21 using Zeeman effect observations, revealing a magnetic field weaker than expected and not dynamically dominant, thus informing star formation models.

Contribution

First direct measurement of magnetic fields in the dense PDR of DR 21 using OH Zeeman effect, combining multiple lines to analyze physical conditions and magnetic influence.

Findings

Magnetic field in DR 21's PDR is ~0.13 mG, weaker than density-magnetic field relation predicts.

OH and C+ coexistence indicates origin of absorption lines in the PDR.

Magnetic energy density is not dynamically significant in the current stage of DR 21.

Abstract

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.