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, $\zeta$ Oph, and hosts a
magnetic field that is coherent across parsec-length scales. Near-infrared
$H$-band (1.6$\mu$m) linear polarization measurements were obtained for 3,896
background stars across a $1\deg \times 1.5\deg$ 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 $\zeta$ 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 $\zeta$ Oph has compressed the magnetic field on the
western edge of L204. The plane-of-sky magnetic field strength is estimated to
be $\sim 11 - 26$ $\mu$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 $\zeta$ 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 $\zeta$ Oph. Both findings highlight the importance of
external illumination, here from $\zeta$ Oph, in aligning dust grains to
embedded magnetic fields.
