Near-infrared imaging polarimetry of bipolar nebulae - I. The BN-KL region of OMC-1

New high-spatial-resolution polarization images of the BN–KL region of OMC-1 from 1.25–3.6 μm are presented. At the longer wavelengths these show a centro-symmetric polarization vector pattern, centered mainly on IRc2, and high degrees of polarization across the nebula, confirming that the diffuse nebulosity is dominated by the scattering of radiation, mainly from IRc2. Degrees of polarization, position angles and magnitudes are given for the observable IRc sources; these are discussed. The Hubble relation has been used to determine the extinction along the line-of-sight to 1Rc2, which is found to be > 9 mag at 2.2 μm and 6 mag at 3.6 μm. Color maps show that the nebula becomes bluer with increasing angular distance from IRc2, consistent with decreasing levels of extinction away from the star. Percentage polarization values increase with wavelength for most regions of the nebula and this is interpreted as due to the presence of grains substantially larger than those found in the diffuse ISM. Modeling of the observed 2.2-μm polarization values across the northwestern outflow is consistent with the outflow geometry being a parabolic cavity, with the scattering of radiation taking place preferentially from the surface of this cavity. In this model the outflow is inclined by no more than 30 ° from the plane-of-the-sky.