Statistics – Applications
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21533201v&link_type=abstract
American Astronomical Society, AAS Meeting #215, #332.01; Bulletin of the American Astronomical Society, Vol. 42, p.430
Statistics
Applications
Scientific paper
Chandrasekhar & Fermi (1953) used the dispersion of starlight polarization vectors about contours of Galactic latitude to determine the strength of the magnetic field in the arms of the Galaxy. The same technique, the Chandrasekhar-Fermi (CF) method, has been applied to estimates of field strengths in the relatively dense medium of molecular clouds. The basis for deriving field strengths from dispersion measurements is the same for observations of Galactic arms or molecular clouds: in either case dispersion decreases as the field strengthens. But in the case of the Galactic arms, the dispersion is due to magnetohydrodynamic (MHD) waves; the displacements are perpendicular to the direction of propagation. In the case of turbulent dispersion in molecular clouds, there is no preferred direction. The turbulent component can be in any orientation and may have structure due to effects such as differential rotation, gravitational collapse, or expanding H II regions. Consequently, dispersion measured about mean fields, assumed straight, may be much larger than should be attributed to MHD waves or turbulence. Dispersion measured about model large-scale fields that give approximate fits to a polarization map will result in better estimates but still give inaccurate values of the turbulent component.
Here we describe a method for determining magnetic field dispersion about local structured fields, without assuming any model for the large-scale field. To do this, we use the second-order structure function of the measured polarization vectors to separate the turbulent component of the dispersion from the large-scale field. Our study incorporates the effect on the measured dispersion of signal integration through the thickness of the cloud as well as across the area subtended by the telescope beam. Our method provides accurate, independent estimates of the turbulent to mean magnetic field strength ratio. We discuss applications to the molecular clouds Orion, M17, and DR21.
Hildebrand Roger H.
Houde Martin
Vaillancourt John E.
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