Magnetic fields in star-forming clouds: How can first contribute?

The SCUBA polarimeter at the James Clerk Maxwell Telescope has been used to probe the magnetic field geometry within the OMC-3 region of the Integral Filament of Orion A, the Barnard 1 cloud in Perseus and the B2 core in the ρ Ophiuchus dark cloud. In the submillimetre, polarized dust emission arises from rapidly spinning dust grains aligned by the local magnetic field. Although the polarized emission from each grain is orthogonal to the local field direction, a variation of the magnetic field orientation through the cloud can produce complex polarization patterns from which the field geometry cannot easily be determined without modelling. In each of the regions observed, the polarization patterns are inconsistent with strictly uniform or unidirectional magnetic fields on extended spatial scales. In each case, a decrease in polarization percentage is observed toward higher total intensities. In OMC-3, we have modelled the polarization pattern as arising from a bent filamentary cloud threaded by a helical magnetic field. The model is remarkably consistent with the polarization pattern observed.