The SIMPLIFI survey: Polarization of dense filaments with SOFIA. III. Relative orientations of antenna temperature gradients with the magnetic field in the DR21 molecular cloud

We investigated magnetohydrodynamic (MHD) turbulence in the DR21 molecular cloud using far-infrared polarimetry from SOFIA HAWC+ and molecular spectroscopy from the Nobeyama 45 m telescope. The velocity channel gradient (VChG) method was applied to thin velocity slices of the and J=1-0 emission to determine the antenna temperature gradients and their orientations in the plane of the sky. The relative orientations of the gradients with respect to the magnetic field position angles enabled an evaluation of theoretically predicted alignment of elongated, turbulent eddies for Alfvénic, fast and slow modes, and self-gravity. The projected Rayleigh statistic was calculated for both the full cloud and six regions of interest to provide a quantitative assessment of these alignments. Our analysis finds mostly parallel alignment of the gradient direction and the local magnetic field orientation in the main ridge of DR21 that comprises most of the cloud mass. While parallel alignment can result from fast MHD modes or cloud collapse, previous studies show spectroscopic evidence for the large scale collapse of the DR21 cloud, which supports self-gravity as the primary cause of the observed parallel alignment. One of the six sub-filaments that are connected to the main ridge exhibits mostly perpendicular alignment of the position angles of the gradient and magnetic field. These perpendicular relative orientations may be a product of Alfvén and slow modes in a magnetized, sub-Alfvénic medium even if the sub-filament is accreting onto the main ridge.