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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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