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MIRA - Spectropolarimetry

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About this observation
Observer
john simpson
Time of observation
02/02/2019 - 19:40
Object
MIRA
Observing location
SW Scotland
Equipment
Celestron C8
Star Analyser
WollastonPrism
ATIK Titan
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MIRA passed it’s maximum on 11th Dec. 2018. The image shows the polarisation spectrum recently obtained with an ATIK Titan camera using a Star Analyser coupled to Wollaston Prism on Celestron C8. The dispersion is approx. 15 Angstrom/px with resolution approximately  50 Angstrom at Hα.  Polarisation spectral measurements taken at 0, 45, 90, 135 degree rotation w.r.t  North celestial meridian [approx. 100 each rotation angle] have been combined and wavelength calibrated using Christian Buil’s ISIS

The linear polarisation raw measurements and their average in 100 Angstrom bands show average values steadily increasing from around 0.3% at red wavelengths to  3% at blue. Over the range 4250-7250 Angstrom the wavelength dependence power law fit is close to λ⁻⁴ [strong evidence for  Rayleigh scattering at these wavelengths].  The peaks and troughs in polarisation also  follow the peaks & troughs associated with the TiO spectral bands. There looks to be a small increase in polarisation towards red which is more evident over the full wavelength range up to 9000 Angstrom, but this could easily be just measurement noise. Derived polarisation angles show some wavelength dependence too, but with significant oscillation about mean values that decrease from around 105 deg. at 4250 Angstrom to around 60 deg at 8500 Angstrom.

Forty years ago, McLean & Coyne [Astrophysical Journal, 226, December 1978] reported observations of a similar nature plus importantly  the discovery of highly polarised Balmer emission lines – I’ve no hope of achieving this sort of measurement with my setup.

 

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