Elliptical Polarimetry of Eleven Luminous Late-Type Variables
Ph.D. Thesis, University of Pennsylvania, 1991
Bruce D. Holenstein
Robert H. Koch
Instrumentation was developed to measure the normalized V Stokes vector coincident with the Q and U Stokes vectors of the filtered radiation field from a celestial source. The hybrid method developed to calibrate the system used null-polarization standards for the system offset and linear polarization standards along with a quarter-wave-plate-induced rotation on the Poincare sphere for the system gain.
An observing program was completed that concentrated upon eleven luminous late-type variables (LLTV’s). The program objects are 119 CE tau, Alpha Ori, 6 BU Gem, Psi1 Aur, 72 Leo, V CVn, Alpha Sco, Alpha1 Her, Mu Cep, VV Cep, and Beta Peg. The data for the program objects show circular polarization behavior that is as rich and as complex as was already known for the linear polarization.
The infrared excesses of the the LLTV’s were used to place useful limits on the dust shell particle numbers and masses. The calculations were accomplished by using the stellar flux at visible wavelengths to estimate and remove the stellar flux at infrared wavelengths.
A Monte Carlo software model was developed to aid in the explanation of the observations. The model components include contributions from limb darkening, stellar spots, Rayleigh/Thomson scattering in the stellar atmosphere, Mie scattering from multiple particle types in the dust shell, and interstellar birefringence. The output of the software is an ensemble of maps which display the Stokes vectors and relative flux expected when looking at the modeled object from any direction.
It was concluded that the linear polarization for most of the program objects arises from Rayleigh and Thomson scattering in the atmosphere of the objects and from single scattering by small silicate grains in the dust shells. The bulk of the circular polarization arises from three mechanisms: (1) single scattering of light linearly polarized in the stellar atmosphere and subsequently Mie scattered by spherical or irregularly-shaped aligned particles; (2) magneto-emission arising from spots; and (3) interstellar birefringence acting on an intrinsic stellar linear polarization.
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