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Planetary Systems in Polarized Light: Debris Disk Observations and Instrumentation

Maxwell A. Millar-Blanchaer

Doctor of Philosophy 2016
Graduate Department of Astronomy and Astrophysics, University of Toronto

Understanding planet formation is one of the major challenges of modern astronomy. Polarimetry is a powerful tool with which we can confront this challenge. In particular, polarimetric observations can be useful for imaging debris disks and characterizing exoplanet atmospheres. With that in mind, this thesis has been constructed with two main aspects: i) observational studies of two debris disk systems, β Pic and HD 157587, using the Gemini Planet Imager and ii) the characterization and testing of a new type of diraction grating, called a polarization grating, that we plan to use for future observations of exoplanet atmospheres.

The Gemini Planet Imager is a high-contrast imager that includes a polarimetry mode designed to image circumstellar disks. Here we detail the development of new data analysis techniques that reduce systematics and noise in processed GPI data. We apply these techniques to observations of the β Pic and HD 157587 debris disks and then t each disk image to a geometric disk model. The β Pic disk model's morphology cannot be explained by interactions with the planet β Pic b, and the presence of a second planet could be invoked to explain the discrepancy. In the case of HD 157587, the disk model's geometric centre is oset from the location of the star, which could be explained by a perturbing planet. Characterization of the planets' interactions with their debris disks is a critical method to gain more information about these two systems.

The second component of this thesis focuses on polarization gratings, thin lm optical devices that can simultaneously act as polarizing beam splitters and as spectral dispersive elements. Moreover, they can be designed for high diffraction effciency across a broad wavelength range. These features make polarization gratings useful for many types of astronomical observations. We have carried out laboratory and on-sky test observations using a polarization grating optimized for visible wavelengths. The laboratory tests confirm the expected diffraction effciency and beam splitting capabilities of the grating. Our on-sky observations demonstrate the grating's ability to measure linear polarization fraction and position angle, and recover spectra in an astronomical setting. In the future we plan to use a nearinfrared polarization grating to search for spectropolarimetric features in exoplanet atmospheres.

Reproduced with permission. library@astro.utoronto.ca
September 29, 2016