Magnetic fields in star-forming molecular clouds. II. The depolarization effect in the OMC-3 filament of Orion A

Polarized 850 mum thermal emission data of the region OMC-3 in the Orion A molecular cloud are presented. These data, taken in 1998 with the SCUBA polarimeter mounted on the James Clerk Maxwell Telescope, have been rereduced using improved software. The polarization pattern is not suggestive of a uniform field structure local to OMC-3, nor does the orientation of the vectors align with existing polarimetry maps of the OMC-1 core 20' to the south. The depolarization toward high-intensity regions cannot be explained by uniform field geometry except in the presence of changing grain structure, which is most likely to occur in regions of high density or temperature (i.e., the embedded cores). The depolarization in fact occurs along the length of the filamentary structure of OMC-3 and is not limited to the vicinity of the bright cores. Such a polarization pattern is predicted by helical field models for filamentary clouds. Along similar to 75% of the filament's length, the polarization vectors correlate strongly with the filament axis, a signature of a toroidally dominated helical magnetic field; however, near the southern cores, the vectors are offset in direction by 90 degrees from the gas structure of the integral-shaped filament, as traced by dust. We present three scenarios to explain the observed polarization pattern of OMC-3 in terms of a helical field geometry. A helical field that is toroidally dominated in the north and poloidally dominated in the south could wrap the filament. A crisscrossing of two filamentary structures could produce the observed offset in polarization vectors, or the filament could be bent into the plane of the sky. Qualitative models incorporating a helical field geometry are presented for each of the latter two cases.