A polarimetric study of magnetic fields in star-forming molecular clouds

Abstract

<p>I have used a recently commissioned, highly sensitive polarimeter at the James Clerk Maxwell Telescope to probe the magnetic field geometries of six star-forming regions within 500 pc of the Sun. For five of the regions, these are the first data produced of emission polarization from dust grains aligned by local magnetic fields. The variations in the polarization pattern across the clouds have been compared to predicted patterns from various models of magnetized molecular clouds. In several regions, particularly OMC-3 in the Orion A cloud, the polarization data are very consistent with predictions of a recently developed model of filamentary molecular clouds threaded by helical fields. The regions of NGC 2068 and LBS 23N could also contain helical magnetic field geometries. Although NGC 2024 is successfully modeled by a helical field, this geometry is not consistent with existing data of the line-of-sight magnetic field, which is not probed by polarization measurements. Instead, I suggest that the field geometry in NGC 2024 is that of an expanding ionization front from the associated HII region, bent around the dense ridge of star-forming cores. Prior to this work, most regions were thought to contain an ordered magnetic field component which was essentially unidirectional. Only one region in my sample contains a distribution of polarization vectors which could support such a geometry; in this cloud, Barnard 1, I have estimated the three-dimensional field strength and orientation associated with this "uniform" field and find a significant fraction of the field lies in the plane of the sky. This solution applies only to low column densities and not to the denser cores within Barnard 1, which do not exhibit alignment with the low density material. In short, this work reveals that unidirectional field geometries are not supported on intermediate scales within molecular clouds. Furthermore, no single magnetic field geometry is applicable in all molecular clouds; in each region, local environments and the associated physics must be taken into account.</p>