Polarimetry of young stellar objects - III : Circular polarimetry of OMC-1

Abstract

We present the first imaging circular polarimetry of the Orion Molecular Cloud, OMC-1. The observations, taken in the J, H, K 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 band, conforming to the typical quadrupolar patterns that have been observed in other outflow sources. Overlying this pattern are regions of relatively high degrees of circular polarization to the east and west of the source IRc2, with degrees as high as +17 per cent in the K band, the highest circular polarization yet measured for any young stellar object No circular polarization 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 polarization and regions of enhanced linear polarization, and argue that these observations 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 polarization) suggests that the grains are composed of silicate and/or organic refractory material, and that grains larger than are typically found in the interstellar medium are needed. The lower, background, circular polarization is produced by scattering off randomly oriented grains in the outflow 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 polarization separate from, but near to, the main outflow region. Those regions 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.