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Spectropolarimetry - zeta TAU

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About this observation
Observer
john simpson
Time of observation
18/11/2018 - 23:40
Object
HD37202 [zeta TAU]
Observing location
SW Scotland
Equipment
Celestron C8
Star Analyser
WollastonPrism
ATIK Titan
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The image shows the ‘o’ and ‘e’ polarisation spectra of zeta TAU - a well researched Be star, taken near the end of 2018 with a Star Analyser coupled to Wollaston prism. The linear polarisation arises as a result of electron scattering in a flattened equatorial disc of gas surrounding the star. When scattered, the unpolarised light becomes linearly polarized normal to the plane containing the incident and scattered radiation. The magnitude of the polarisation gives information about the extent of the scatterers and the scattering source and the direction of polarisation gives information about the system geometry – in this case the orientation of the disc.

Only partly successful.  Over a limited wavelength range the processed spectra do however show some typical features associated with Be stars; an angle of polarisation that is roughly constant at 34 degrees across the spectrum whereas the normalised polarisation magnitude displays a dependence on wavelength, dropping in this case from around 1.6% near the blue end to 1.2% near red. Convolving the spectra with a synthetic V-Band filter in ISIS yields a magnitude of 1.3% and polarisation angle of 34.5 degrees.  These compare reasonably well with values of 1.4% and 34.4 degrees measured earlier in 2018  using a Wollaston Prism and Johnson-Cousins V-Band filter in a broadband dual polarimeter configuration.

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