Molecular hydrogen polarization images of OMC-1

A polarization image, with 0″.6 pixel scale, of the molecular hydrogen v = 1-0 S(1) line at 2.1218 μm in the core of the OMC-1, is presented. The line emission is highly clumped, with a prominent filamentary structure protruding to the NW, away from the source of the outflow. There are two main characteristics of the line polarization : a region of aligned vectors over the molecular outflow region of the source, and parallel to the outflow axis, surrounded by a centrosymmetric pattern of vectors, centered on the region of peak molecular hydrogen line emission ("peak 1"), characteristic of scattering. The outflow is thus surrounded by a molecular hydrogen reflection nebula. The aligned vectors arise from the passage of the line photons through a medium of aligned grains between us and the outflow, and the polarization vectors trace the magnetic field direction over the outflow region in the core of the cloud. The mean field direction in the core is parallel to the largescale field direction as derived from far-IR continuum measurements (e.g., Hildebrand et al.). The filament to the NW is dichroically polarized, indicating that the line emission is intrinsic to it, and not simply arising from increased scattering. Near IRc2 there is a significant twist in the aligned vectors, with the magnetic field direction changing close to the source of the outflow. From measurements of the ratio of the 1-0 S(1) line at 2.12 μm and the 1-0 Q-branch at 2.4 μm in the reflection nebula, we conclude that the grain size there is typical of the diffuse interstellar medium. In contrast, the grains in the cloud core are larger (e.g., Minchin et al.), and thus it appears that the grain size decreases with distance from the cloud core.