We present polarization observations in DR21 ( OH ) from thermal dust emission at 3 mm and from CO J= 1 \rightarrow 0 line emission .
The observations were obtained using the Berkeley-Illinois-Maryland Association array . Lai et al .
( 26 ) observed this region at 1.3 mm for the polarized continuum emission , and also measured the CO J = 2 \rightarrow 1 polarization .
Our continuum polarization results are consistent with those of Lai et al .
( 26 ) .
However , the direction of the linear polarization for the J = 1 \rightarrow 0 is perpendicular to that of the CO J = 2 \rightarrow 1 polarization .
This unexpected result was explored by obtaining numerical solutions to the multilevel , radiative transfer equations for a gas with anisotropic optical depths .
We find that in addition to the anisotropic optical depths , anisotropic excitation due to a source of radiation that is external to the CO is needed to understand the orthogonality in the directions of polarization .
The continuum emission by dust grains at the core of DR21 ( OH ) is sufficient to provide this external radiation .
The CO polarization must arise in relatively low density ( n _ { H _ { 2 } } \sim 100 cm ^ { -3 } ) envelope gas .
We infer B \sim 10 \mu G in this gas , which implies that the envelope is subcritical .