Near - Infrared Imaging Polarimetry of the Star Forming Region NGC 2024

We conducted wide-field JHKs imaging polarimetry toward NGC 2024, which is a massive star-forming region in the Orion B cloud. We found a prominent and extended polarized nebula over NGC 2024, and constrained the location of illuminating source of the nebula through the analysis of polarization vectors. A massive star, IRS 2b with a spectral type of O8 − B2, is located in the center of the symmetric vector pattern. Five small polarized nebulae associated with YSOs are discovered on our polarization images. These nebulae are responsible for the structures of circumstellar matter (i.e., disk/envelope systems) that produce strongly polarized light through dust scattering. For the point-like sources, we performed software aperture polarimetry in order to measure integrated polarizations, with which we detected candidate sources associated with circumstellar material by selecting sources with a larger polarization than that estimated from the extinction of foreground material. We investigated the fraction of highly polarized sources against the intrinsic luminosity of stars (∝ mass), and found that the source detection rate remains constant from low (brown dwarfs) to higher luminosity (solar-type) stars. This result indicates the relative disk scale-hight is rather independent of the stellar mass. We confirmed the result using our polarimetry of the stars with known spectral types in NGC 2024. We found five young brown dwarfs with highly polarized integrated emission. These sources serve as direct evidence for the existence of disk/envelope system around brown dwarfs. We investigated the magnetic field structure of NGC 2024 through the measurements of dichroic polarization. The average position angle of projected magnetic fields across the region is found to be 110. We found a good consistency in magnetic field structures obtained using near-infrared dichroic polarization and sub-mm/far-infrared dust emission polarization, indicating that the dichroic polarizations at near-infrared wavelengths trace magnetic field structures inside dense (AV <∼ 50 mag) molecular clouds.