Near-infrared imaging polarimetry of bipolar nebulae 1:the BN-KL region of OMC-1

Abstract

New high-spatial-resolution polarization images of the BN-KL region of OMC-1 from 1.25-3.6 microns 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. 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-micron 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 deg from the plane-of-the-sky.