The Effect of Shear Flows on the Davis-Chandrasekhar-Fermi Approximation

The Davis-Chandrasekhar-Fermi (DCF) method is one of the most common means to estimate the magnetic field strength from dust polarimetric observations. Its physical foundation lies on the idea that the speed of an Alfvén wave is determined by the amplitude of the turbulent motions in the gas. However, this scenario does not consider the large-scale motions of the gas such as shear flows, which often is evident in polarimetric data. We extended the DCF method to include such effects by studying the propagation of an Alfvén wave in a medium with a background structured flow. The new approximation was first tested on synthetic polarization in order to determine the range of physical variables (i.e. mass density, turbulent velocity, shear-flow amplitude) in which it is valid. Finally, the extended DCF approximation was used to determined the strength of magnetic field in the Circus-nuclear Disk (CND) in the galactic center for which polarimetric data was obtained with HAWC+/SOFIA.