Abstract
In astronomy, high-contrast imaging is a rapidly developing field focussing on the observation of faint targets in close proximity to very bright ones. This can be applied to the direct observation of exoplanets, but also for observations of disks or dust shells surrounding stars. The brightness of the planet Jupiter,
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if observed from a nearby star, is 10-9 times that of the Sun. The contrast required to observe extended circumstellar sources is however several orders of magnitude less. On current 8-m class telescopes with modern adaptive-optics systems, coronagraphic imagers achieve a contrast of 10-5 at 1 arcsecond from the star. As a result, only a handful of very bright exoplanets at fairly large angular separations from their parent stars have been observed directly. Polarimetry offers an attractive alternative to coronagraphy: Starlight scattered by circumstellar material will become linearly polarized to some degree, allowing us to distinguish it from the unpolarized starlight. Polarimetry has additional benefits: The degree of polarization at a given wavelength depends on both the scattering particle size and the scattering angle. Multi-wavelength polarimetry therefore allows us to study both the composition and the structure of the scattering medium. Moreover, polarimetry does not exclude coronagraphy. The two techniques can be combined to achieve very high contrasts. This thesis documents the design, performance and first science results of ExPo, a sensitive imaging polarimeter. ExPo has been developed by the Utrecht University Astronomical Institute and operates at visible wavelengths with a field of view of 20 arcseconds. ExPo does not include an adaptive optics system and the resolution is therefore seeing-limited. The pixel scale is 0.08 arcseconds per pixel. ExPo is implemented as a modulated dual-beam polarimeter to reduce systematic errors. The modulation frequency, limited by the maximum frame rate of the camera, is 35Hz. This is sufficient to minimize the effect of atmospheric seeing change during individual exposures. ExPo has been used on the William Herschel Telescope for a number of successful observing runs targeting young (Herbig Ae/Be) stars known or suspected to harbour protoplanetary disks. A separate observing run was dedicated to observing dust shells around evolved (post-AGB) stars. On sky, a sensitivity of 10-4 is achieved for a MV = 7.7 star in a 10 minute accumulated exposure. The contrast at 1 arcsecond separation is 10-6. The performance is photon-limited. By using longer exposure times the sensitivity, and thus contrast, can be increased. The calibrated polarimetric accuracy of the observations is usually better than 1%. First science results include high-contrast observations of the disk around AB Aurigae, in which several spiral arms are detected. Analysis of our polarimetric imaging of the dust envelope surrounding Mira indicates that it is highly bipolar in shape and consists of silicate-rich dust with an upper particle size limit of 0.05 μm. In a laboratory experiment simulating the use of ExPo in combination with an adaptive optics system and a coronagraph, we achieve a contrast of 10-8 at 1 arcsecond from the star, approaching the contrast needed to directly observe Jupiter-like exoplanets.
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