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Paper: Polarimetry of the Exoplanet Host Systems τ Boötis and 55 Cancri with PLANETPOL
Volume: 449, Astronomical Polarimetry 2008: Science from Small to Large Telescopes
Page: 371
Authors: Lucas, P. W.; Hough, J. H.; Hirst, E.; Harrison, D.; Bailey, J. A.; Tamura, M.
Abstract: Planetpol is an aperture polarimeter that was designed to search for reflected light from extrasolar planets with short orbital periods. It is sensitive to fractional linear polarizations of ∼10–6 for nearby bright stars. We present polarimetry of two of the brightest exoplanet systems, τ Boo and 55 Cnc. No periodic changes in polarization are detected that are consistent with those expected for the known planets τ Boo b and 55 Cnc e. The standard deviations in the night to night average Stokes Q/I and U/I polarization parameters are 5.1× 10–6 for τ Boo and 2.2× 10–6 for 55 Cnc. The larger scatter of the τ Boo data may be related to the magnetic activity of the star. We use Monte Carlo multiple scattering models of uniform semi-infinite atmospheres with Rayleigh-like scattering properties to investigate the relation between the broad band geometric albedo of extrasolar planets and the polarization. The single scattering albedo is treated as a free parameter to simulate the effect of absorption by dust particles or molecular absorption. Subject to these assumptions we place 4-σ upper limits on the geometric albedo of AG<0.37(R/1.2 RJup)–2pm–1 for the “hot Jupiter'' τ Boo b and AG<0.13(R/1.2 RJup)–2pm–1 for the “hot Neptune'' 55 Cnc e, where pm is the maximum single scattering polarization at 90°. The non-detections for these planets contrast markedly with the recent report of a 3σ detection of a periodic polarization signal from the active star HD 189733 with amplitude P = 2 × 10–4, attributed to the planet HD 189733 b.
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