Statistics – Applications
Scientific paper
May 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt........13r&link_type=abstract
Thesis (PhD). UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN, Source DAI-B 60/09, p. 4658, Mar 2000, 144 pages.
Statistics
Applications
6
Scientific paper
The goal of this thesis was to be able to study the role of magnetic fields in the star formation process. The direction of the plane of sky component of the magnetic field can be obtained by mapping the linear polarization of dust emission from aligned dust grains. The most widely accepted mechanism for the alignment is the one originally proposed by Davis and Greenstein in which paramagnetic relaxation of spinning grains causes them to rotate with their long axes perpendicular to the direction of the magnetic field. The dust emission from these aligned grains is parallel to the long axes and perpendicular to the magnetic field. In order to achieve the stated objective, we designed and built a polarimetry system for the ten-element Berkeley Illinois Maryland Association (BIMA) array. The active elements of this system were achromatic quarter wave plates that produced circular polarization from linear. Cross-correlation of orthogonal circular polarization is the most sensitive way for an interferometer to detect linear polarization in the sky. The polarimetry systems were designed to work at 90 GHz (3.3 mm) and 230 GHz (1.3 mm). The sensitivity to dust emission increases with frequency in the millimeter wavelength regime. One of the prime objects that was chosen for this study is the star forming region in Orion BN/KL. Previous polarimetric studies with lower resolution have shown that the polarization direction (and the field) is uniform across the cloud but with a decrease in the degree of polarization towards BN/KL. The BIMA observations, which have a resolution of approximately 4 '', show that this decrease in polarization is due to considerable structure in the polarization vectors. However, it must be pointed out that this structure in the polarization does not necessarily imply that the field is twisted. It is possible that alternative grain alignment mechanisms might cause this observed change in the polarization and that the field is relatively uniform. Other applications of polarimetry at millimeter wavelengths include imaging of radio galaxies (for e.g. Cygnus A, M87), monitoring the time variability of polarized SiO maser emission (RLeo, Orion IRc2) and studying the linear polarization of quasars.
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