High-Angular Resolution Dust Polarization Measurements: Shaped B-field
  Lines in the Massive Star Forming Region Orion BN/KL
  We present observational results of the thermal dust continuum emission and
its linear polarization in one of the nearest massive star-forming sites Orion
BN/KL in Orion Molecular Cloud-1. The observations were carried out with the
Submillimeter Array. With an angular resolution of 1" (~2 mpc; 480 AU), we have
detected and resolved the densest cores near the BN/KL region. At a wavelength
of ~870 micron, the polarized dust emission can be used to trace the structure
of the magnetic field in this star-forming core. The dust continuum appears to
arise from a V-shaped region, with a cavity nearly coincident with the center
of the explosive outflows observed on larger scales. The position angles
(P.A.s) of the observed polarization vary significantly by a total of about 90
degree but smoothly, i.e., curl-like, across the dust ridges. Such a
polarization pattern can be explained with dust grains being magnetically
aligned instead of mechanically with outflows, since the latter mechanism would
cause the P.A.s to be parallel to the direction of the outflow, i.e.,
radial-like. The magnetic field projected in the plane of sky is therefore
derived by rotating the P.A.s of the polarization by 90 degree. We find an
azimuthally symmetric structure in the overall magnetic field morphology, with
the field directions pointing toward 2.5" west to the center of the explosive
outflows. We also find a preferred symmetry plane at a P.A. of 36 degree, which
is perpendicular to the mean magnetic field direction (120 degree) of the 0.5
pc dust ridge. Two possible interpretations of the origin of the observed
magnetic field structure are discussed.
