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 3896 background stars across a 1 degrees x 1 degrees.5 region centered on the dense Cloud 3 in L204, using the Mimir near-infrared instrument on the 1.8 m 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 similar to 11-26 mu G using the Chandrasekhar-Fermi method. The polarimetry data also reveal that the polarization efficiency (PE = P-H/A(V)) steadily decreases with distance from zeta Oph (-0.09% +/- 0.03% mag(-1) pc(-1)). Additionally, power-law fits of PE versus A(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.