HCO+Spectropolarimetry and Millimeter Continuum Polarimetry of the DR 21 Star‐forming Region

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Abstract
We report the results of a search for linear polarization in emission lines toward the outflow lobes of DR 21 (HCO+J = 1-0), Mon R2 (HCO+ J = 1-0), and IRAS 16293-2422 (CS J = 2-1). Theoretical models suggest linear polarization in optically thin emission lines may be present at the level of a few percent in these regions. However, no polarization was detected. An upper limit of 0.4% can be placed on the polarization toward DR 21 and Mon R2, and 1.2% toward IRAS 16293-2422. Using published HCO+ interferometry of DR 21, we argue that the lack of polarization is probably due to the presence of clumpy, turbulent gas, which is unresolved by our beam (70'' FWHM). Diminished polarization by multiple scattering cannot be ruled out. We also present new λ = 1.3 mm and λ = 2 mm continuum polarimetry of the DR 21 dust core. Polarizations of P = 1.7% ± 0.2% and P = 0.77% ± 0.13%, respectively, were detected. The λ = 2 mm polarization is lower than the ~2.0%-2.5% previously reported at millimeter and submillimeter wavelengths. However, with the unpolarized free-free component of the flux density from H II regions in the star-forming core removed, the level of polarization is consistent with the previous observations. Based on our observations and those previously reported, there is no clear dependence of level of polarization on beam size in the inner ~30'' of the dust core. The λ = 2 mm polarization position angle, 2° ± 5°, is rotated ~20° with respect to the previous measurements. If the rotation is real, it implies the magnetic field on this expanded scale (42'' FWHM beam) is more perpendicular to the general elongation of the dust cloud than it is within the inner 20'' of the dust core.