Polarimetry of young stellar objects { III

We present the rst imaging circular polarimetry of the Orion Molecular Cloud, OMC-1. The observations, taken in the J , H , K n and nbL bands, reveal a complex pattern of circular polarisation. Globally, there is a background circular polarisation of the order of 2 percent in the K n band, conforming to the typical quadrupolar patterns that have been observed in other outtow sources. Overlaying this pattern are regions of relatively high degrees of circular polarisation to the east and west of the source, IRc2, with degrees as high as +17 percent in the K n band, the highest circular polarisation yet measured for any young stellar object. No circular polarisation is seen in the J band indicating that the circular polarisation detected at longer wavelengths originates from within OMC-1 and not from scattering oo the foreground ionisation front associated with the M42 nebula. We demonstrate a correlation between these patches of high circular polarisation and regions of enhanced linear polarisation and argue that these observations are best explained using a model which incorporates scattering of radiation oo oblate grains, which have been aligned by the local magnetic eld. Modelling of the ellipticity (the ratio of circular to linear polarisation) suggests that the grains are composed of silicate and/or organic refractory material and that grains larger than are typically found in the ISM are needed. The lower, background, circular polarisation is produced by scattering oo randomly oriented grains in the outtow cavities, the grain alignment being destroyed by the passage of shocks. We put forward a morphological model for OMC-1 which has the regions of high polarisation separate, but nearby, to the main outtow region. Those regions exhibiting high polarisation must somehow have a direct view of the nebula's illuminating source. Implications of this work to the origins of life are brieey discussed.