Magnetic fields in star formation regions: 1.3 millimeter continuum polarimetry

We present a nine-point λ = 1.3 mm continuum polarization map of the inner arcminute of the DR 21 cloud core. The polarization and position angles are very uniform, and the inferred magnetic field (P.A. ∼ 75°) is nearly orthogonal to the cloud elongation (P.A. ∼ 7°). Applying the virial theorem and comparing the continuum polarimetry, we find that the magnetic field strength must be greater than a few mG to have a significant impact. Turbulent gas motions are probably a more significant source of support against self-gravity in the cloud core than in the magnetic field. We also report a survey of the λ = 1.3 mm polarization of 14 star-forming cloud cores (〈P〉 = 1.6%). The λ = 1.3 mm distribution is similar to the λ = 100 μm and λ = 800 μm polarization distributions in the literature except that the 1.3 mm distribution peaks at P < 1%. We compared our polarimetry of nine of the cloud cores to physical parameters derived from far-infrared photometry in a homogeneous fashion. Consistent with theoretical expectations, the polarizations of these cloud cores do not depend on the λ = 1.3 mm dust optical depth, emission temperature, or emissivity spectral index. Although the sample is very small, it appears that the polarization is larger on average for the cloud cores with mean densities of n^H2 > 1.5 × 10⁷ cm^-3 than for those with n^H2 > 1.5 × 10⁷ cm^-3. The sky-plane projection of the magnetic field lines in the seven elongated cloud cores with 800 μm or 1.3 mm polarization detections greater than 3 σ appear randomly distributed with respect to the position angles of cloud core elongations. This implies that magnetic fields do not provide substantial anisotropic support against self-gravity in this sample of star-forming cloud cores. The magnetic fields in the cloud cores also appear randomly oriented with respect to the Galactic plane.