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 m in the core of the Orion Molecular Cloud One, is presented. The line emission is highly clumped, with a prominent lamentary structure protruding to the NW, away from the source of the out ow. There are two main characteristics of the line polarization; a region of aligned vectors over the the molecular out ow region of the source, and parallel to the out ow axis, surrounded by a centro-symmetric pattern of vectors, centered on the region of peak molecular hydrogen line emission (`Peak 1'), characteristic of scattering. The out ow is thus surrounded by a molecular hydrogen re ection nebula. The aligned vectors arise from the passage of the line photons through a medium of aligned grains between us and the out ow, and the polarization vectors trace the magnetic eld direction over the out ow region in the core of the cloud. The mean eld direction in the core is parallel to the large scale eld direction as derived from far{IR continuum measurements (e.g., Hildebrand et al. 1982). The lament 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 signi cant twist in the aligned vectors, with the magnetic eld direction changing close to the source of the out ow. From measurements of the ratio of the 1{0 S(1) line at 2.12 m and the 1-0 Q-branch at 2.4 m in the re ection nebula, we conclude that the grain size there is typical of the di use interstellar medium. In contrast, the grains in the cloud core are larger (e.g., Minchin et al. 1991a), and thus it appears that the grain size decreases with distance from the cloud core. Subject headings: