Tracing the Magnetic Field in Orion A

Abstract

We use extensive 350 um polarimetry and continuum maps obtained with Hertz and SHARC II along with HCN and HCO+ spectroscopic data to trace the orientation of the magnetic field in the Orion A star-forming region. Using the polarimetry data, we find that the direction of the projection of the magnetic field in the plane of the sky relative to the orientation of the integral-shaped filament varies considerably as one moves from north to south. While in IRAS 05327-0457 and OMC-3 MMS 1-6 the projection of the field is primarily perpendicular to the filament it becomes better aligned with it at OMC-3 MMS 8-9 and well aligned with it at OMC-2 FIR 6. The OMC-2 FIR 4 cloud, located between the last two, is a peculiar object where we find almost no polarization. The projected angle of the field is more complicated in OMC-1 where it exhibits smooth variations in its orientation across the face of this massive complex. By combining the polarimetry and spectroscopic data we were able to measure a set of average values for the inclination angle of the magnetic field relative to the line of sight. We find that the field is oriented quite close to the plane of the sky in most places. More precisely, the inclination of the magnetic field is ~73 deg. around OMC-3 MMS 6, ~74 deg. at OMC-3 MMS 8-9, ~80 deg. at OMC-2 FIR 4, ~65 deg. in the northeastern part of OMC-1, and ~49 deg. in the Bar. We also present polarimetry data for the OMC-4 region located some 13 arcminutes south of OMC-1.