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, Kn 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 Kn 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 Kn 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.

VLT/NACO infrared adaptive optics images of small scale structures in OMC1

“…There is however strong circular polarization in the K-band continuum in the zone shown in Fig. 2 (Chrysostomou et al 2000). Those authors ascribed the origin of circular polarization to scattering from oblate grains oriented in magnetic fields.…”

Observations of spatial and velocity structure in the Orion molecular cloud

“…It has accordingly been studied over a large range of wavelengths from the radio to the X-ray (e.g. Genzel & Stutzki 1989;Wright et al 1996;O'Dell 2001;Chrysostomou et al 2000;Garmire et al 2000;Ferland 2001;Feigelson et al 2002;Doi et al 2002;O'Dell & Doi 2003;Tan 2004) and has become the archetypal site for studies of star formation. Many features are accordingly very well-described, such as the detailed morphology of the HII region associated with θ 1 Ori C, the dominant source of ionizing radiation in the Trapezium (O'Dell 2001).…”

Imaging polarimetry as a diagnostic tool

“…Degrees of CP of up to 17 per cent in the K-band were reported in the OMC-1 region in Orion by Chrysostomou et al (2000), and to date this is the highest degree of CP seen in diffuse nebulosity. Also see Buschermohle et al (2005) for a wide-field CP survey of the OMC-1 region.…”

“…Also see Buschermohle et al (2005) for a wide-field CP survey of the OMC-1 region. Chrysostomou et al (2000) found that high CP occurred in regions where high LP was also seen (although the reverse was not the case) and speculated that the CP resulted from the scattering of infrared light, originating from IRc2, by dust grains aligned in a structured magnetic field within the OMC-1 cloud. A simple model was proposed to show that the required degrees of CP could indeed be produced with the proposed field geometry.…”