Statistics – Computation
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........29w&link_type=abstract
Thesis (PH.D.)--THE UNIVERSITY OF WISCONSIN - MADISON, 1993.Source: Dissertation Abstracts International, Volume: 54-10, Sectio
Statistics
Computation
Polarization
Scientific paper
Ultraviolet spectropolarimetric observations have provided data on the wavelength dependence of interstellar polarization along eight sightlines. These data allow us to take a new look at dust grain models which have been used to fit the extinction and polarization in the visible and infrared. The eight sightlines show three different interstellar polarization wavelength dependencies: Serkowski, super-Serkowski, and a bump at 0.2175 mu m. The Serkowski and super-Serkowski curves can be fit by changing the size distributions in a particular way. These fits imply that there should be a correlation between curve type and lambda_{max }. Additionally, we find that the failure of the composite grain model of Mathis & Whiffen seems to raise questions about the applicability of Effective Medium Theory (EMT). The need for EMT and its rather restrictive assumptions may be circumvented through a direct computation of the scattering properties via finite element methods, such as the Discrete-Dipole Approximation (DDA). Extensive calculations with the DDA method are used here to examine more directly the effects of porosity. For both solid and porous targets we establish both numerical and physical convergence properties over the range of size parameter that is required for our study. DDA cross sections for grains with a range of porosity are compared to those computed by the EMT/series expansion technique to examine the applicability of several mixing rules, including two extensions of the Bruggeman rule. We show that for particles with Rayleigh vacuum inclusions the extension proposed by Rouleau & Martin is quite successful. We also investigate the effects of larger, non-Rayleigh vacuum inclusions for various levels of porosity and find that they can be significant. Finally, we investigate the existence of wavelength structure in the interstellar polarization curve. Very high signal-to-noise spectropolarimetric observations were made for nine moderately reddened sightlines. These data reveal the absence of structure down to very low levels of polarization. This refutes the claims of structure by Wolstencroft & Smith for three of our sightlines. Furthermore, the data provide excellent constraints for theoretical modeling efforts.
Wolff Michael Joseph
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