Magnetic fields in star-forming molecular clouds. I. The first polarimetry of OMC-3 in Orion A

The first polarimetric images of the OMC-3 region of the Orion A filamentary molecular cloud are presented. Using the new imaging polarimeter on the Submillimeter Common User Bolometric Array at the James Clerk Maxwell Telescope, we have detected polarized thermal emission at 850 mu m from dust along a 6' length of the dense filament. The polarization pattern is highly ordered and is aligned with the long axis of the filament throughout most of the region, diverging only near the southern boundary by 30 degrees-50 degrees. If the polarization arises from thermal emission of dust grains aligned via either paramagnetic inclusions or radiative torques, this configuration indicates a plane-of-sky magnetic field which is normal to the filament along most of its length. The mean percentage polarization is 4.2% with a 1 sigma dispersion of 1%. This region is part of the integral-shaped filament, and active star formation is ongoing along its length, with only two of nine dust condensations in our held lacking evidence of outflow activity. The outflow directions do not appear to be consistently correlated with the direction of the plane-of-sky field or the filament structure itself. Depolarization toward the filament center, previously detected in many other star-forming cores and protostars, is also evident in our data.