Mid- and far-infrared polarimetric studies of the core of OMC-1: The inner field configuration

We present imaging polarimetry of the central 30 arcsec of OMC-1 at 12.5 and 17 mu m with arcsecond resolution, together with complementary spectropolarimetry in the ranges 8-13 and 16-22 mu m at selected positions, and polarimetry at 800 mu m over an approximately l-arcmin field. The polarization is due to the dichroism of aligned grains in emission in the far-infrared, and predominantly due to absorption in the mid-infrared. The images reveal large variations of polarization fraction and position angle in BNKL, the central region, and these can explain the low fractional polarization observed when this region is unresolved, as in the far-infrared. The mid-infrared polarization indicates that a substantial component of magnetic field is aligned with the plane of the disc-like structures inferred from millimetrewave studies, and suggests the presence of a toroidal field within the disc.