Millimeter-wave polarimetry of star formation regions and evolved stars

Abstract

A new λ = 1.3 mm polarimeter, Cyclops, was constructed to make observations of dust continuum emission from star formation regions. The polarization of the inner arcminute of DR 21 was mapped with Cyclops. The polarization percentage and position angle are remarkably constant, indicating a uniform magnetic field throughout the cloud. Turbulent gas motions are a more significant source of support against self gravity in the cloud core than thermal pressure or magnetic fields. The polarization toward the cloud core increases slightly from λ = 100 μm to λ = 2 mm and is consistent with the standard dust composition of silicates and graphite. A small continuum polarization survey of cloud cores with embedded protostars was made with Cyclops and combined with observations from the literature. There is no clear tendency for any preferred alignment of cloud core elongations with respect to magnetic field lines, especially for the bright, high mass star forming regions. This confirms that the massive cloud cores are magnetically supercritical. The magnetic field lines appear randomly oriented with respect to the local Galactic plane position angles, implying that the random component of the Galactic magnetic field dominates the spiral component in this sample. Three-σ upper limits of 0.4%, 1.2%, and 1.2% were placed on the polarization of the HCO⁺ J = 1-0 emission line from the DR 21 and Mon R2 molecular outflows, and the CS J = 2-1 line from the IRAS 16293-2422 molecular outflow, respectively. These polarizations are an order of magnitude lower than predicted by theoretical models. In the case of DR 21, the lack of polarization is probably due to a disordered magnetic field in clumpy, turbulent gas, although multiple scattering may also diminish the polarization. CS J = 2-1 polarizations of 0.9% ± 0.1% and 5.1% ± 1.5% were observed from the envelopes of the evolved stars IRC+10216 and CRL 2688, respectively. An anisotropic optical depth to escape of infrared photons from the central star, perhaps caused by a toroidal dust distribution, could generate the IRC+10216 polarization.