We present the first imaging circular polarimetry of the Orion Molecular Cl
oud, OMC-1. The observations, taken in the J, H, K-n and nbL bands, reveal
a complex pattern of circular polarization. Globally, there is a background
circular polarization of the order of +/- 2 per cent in the K-n band, conf
orming to the typical quadrupolar patterns that have been observed in other
outflow sources. Overlying this pattern are regions of relatively high deg
rees of circular polarization to the east and west of the source IRc2, with
degrees as high as +17 per cent in the K-n band, the highest circular pola
rization yet measured for any young stellar object. No circular polarizatio
n is seen in the J band, indicating that the circular polarization detected
at longer wavelengths originates from within OMC-1 and not from scattering
off the foreground ionization front associated with the M42 nebula.
We demonstrate a correlation between these patches of high circular polariz
ation and regions of enhanced linear polarization, and argue that these obs
ervations are best explained using a model that incorporates scattering of
radiation off oblate grains, which have been aligned by the local magnetic
field. Modelling of the ellipticity (the ratio of circular to linear polari
zation) suggests that the grains are composed of silicate and/or organic re
fractory material, and that grains larger than are typically found in the i
nterstellar medium are needed. The lower, background, circular polarization
is produced by scattering off randomly oriented grains in the outflow cavi
ties, the grain alignment being destroyed by the passage of shocks.
We put forward a morphological model for OMC-1 which has the regions of hig
h polarization separate from, but near to, the main outflow region. Those r
egions exhibiting high polarization must somehow have a direct view of the
illuminating source of the nebula.
Implications of this work to the origins of life are briefly discussed.