2005ApJ...626..959M

We present the first interferometric observations of linearly polarized emission toward the OMC-3 region of the Orion A cloud. We have observed the MMS 6 protostellar core at 1.3 mm with the Berkeley-Illinois-Maryland Association (BIMA) array, achieving a resolution of 4".3x3".0. We find that the polarization angle measured changes systematically across the core, orienting along a dust extension to the northwest. The polarization angle is oriented similarly to the 850 and 350 µm polarized emission measured by the SCUBA and Hertz polarimeters. A polarization hole is detected, as is typical of polarized emission data toward cores. Since the BIMA data are insensitive to structure on spatial scales of >40", the emission detected is dominated by the core and not the integral-shaped filament in which it is embedded. Observations of CO J=2-1 reveal CO emission potentially associated with the core, but no outflow signature is detected. Utilizing the Chandrasekhar-Fermi method, we have used the dispersion in the polarization vectors to estimate a field strength of 640 µG in the plane of the sky, assuming a corrective Q-factor of 0.5. Applying the recent measurement of the inclination of the field to the line of sight (Houde et al.), a total field strength of 680 µG is derived. Despite highly nonthermal line widths, the kinetic energy density is found to be insufficient to support this core against gravitational collapse. The magnetic energy density, when combined with the predominantly turbulent kinetic energy density, is comparable to the effects of gravity, but its value is highly dependent on the applied Q-factor to a degree that the core may be subcritical or supercritical. The preservation of the field geometry from large to small scales in this core is consistent with observations of a second protostellar core in a filamentary cloud in Orion B.