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F. Heitsch (MPIA), E.G. Zweibel (JILA), P.S. Li (NCSA), M.-M. Mac Low (AMNH), M.L. Norman (UCSD)
Far infrared polarimetry can be used to map magnetic fields in dense clouds, using the thermal emission from aligned dust grains as a tracer. We are making polarimetric maps from numerical simulations to calibrate the effects of line of sight and horizontal averaging on the apparent field morphology.
We use full 3D simulations of self-gravitating MHD turbulence run with ZEUS-3D and ZEUS-MP at resolution up to 5123 to produce simulated polarization maps of molecular clouds. We derive the Stokes parameters U and V by solving the radiative transfer problem according to Zweibel (1995), with the emissivity proportional to the local density. Smoothing due to finite telescope beam size w changes the distribution of polarization angles \sigma(\phi). Doubling w reduces \sigma(\phi) by \approx 25%.
We find that column density structures have arbitrary orientations with respect to apparent magnetic field directions. Examples can be found with apparent field parallel, intermediate and perpendicular to column density filaments.
We test the method of Chandrasekhar & Fermi (1953) to estimate the field strength in a turbulent medium and find that it yields results accurate up to a factor of 2 for field strengths typical for molecular clouds. We suggest a modification which increases the accuracy for weak magnetic fields. We also study the effect of smoothing on the method.
This work is funded in parts by an NSF CAREER fellowship to M-MML, grant number AST 99-85392. Computations were performed at the NCSA and at the Rechenzentrum Garching of the MPG.