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The Magnetic Field of Cloud 3 in L204

The L204 dark cloud complex is a nearby filamentary structure in Ophiuchus North that has no signs of active star formation. Past studies show that L204 is interacting with the nearby runaway O star, $ζ$ Oph, and hosts a magnetic field that is coherent across parsec-length scales. Near-infrared $H$-band (1.6$μ$m) linear polarization measurements were obtained for 3,896 background stars across a $1°\times 1.5°$ region centered on the dense Cloud 3 in L204, using the Mimir near-infrared instrument on the 1.8m Perkins Telescope. Analysis of these observations reveals both large-scale properties and small-scale changes in the magnetic field direction in Cloud 3. In the northern and western $ζ$ Oph facing regions of the cloud, the magnetic field appears to be pushed up against the face of the cloud. This may indicate that the UV flux from $ζ$ Oph has compressed the magnetic field on the western edge of L204. The plane-of-sky magnetic field strength is estimated to be $\sim 11 - 26$ $μ$G using the Chandrasekhar-Fermi method. The polarimetry data also reveal that the polarization efficiency (PE $\equiv P_{\rm H}/A_{\rm V}$) steadily decreases with distance from $ζ$ Oph ($-0.09 \pm 0.03 \% \, {\rm mag}^{-1} \, {\rm pc}^{-1}$). Additionally, power-law fits of PE versus $A_{\rm V}$ for localized samples of probe stars show steeper negative indices with distance from $ζ$ Oph. Both findings highlight the importance of external illumination, here from $ζ$ Oph, in aligning dust grains to embedded magnetic fields.