first detection of polarized scattered light from an exoplanetary atmosphere berdyugina et al....
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First Detection of Polarized Scattered Light from an
Exoplanetary Atmosphere Berdyugina et al. (12/2007)
Florian HerzeleSE Aktuelle Forschung zu Extrasolaren Planeten
WS 07/08
Planetary Atmospheres
• Light scattered in planetary atmospheres is linearly polarized perpendicular to scattering plane
• Characterized by Stokes parameters q and u, normalized to total flux
• During revolution scattering angle changes = Stokes parameters vary
Determination of orbital parameters
System HD189733Name HD 189733 b
Discovered in 2005
M.sin i 1.15 (± 0.046) MJ
Semi major axis 0.0312 (± 0.0004) AU
Orbital period 2.2185733 (± 2e-05) days
Eccentricity 0
Radius 1.156 (± 0.032) RJ
Ttransit 2453988.80336 (± 0.00024)
Inclination 85.76 (± 0.29) deg.
Update 18/04/07
What they did and how
• Observations in 2006-2007 with double image CCD polarimeter DIPol @ remotely controlled 60cm KVA telescope on La Palma
• Rotating superachromatic plate as retarder and calcite plate as analyzer
• Cylces of 16 exposures (retarder rotated at 22.5°)
When they did it
• 2006: 10-15s exposures @ 2x16 positions; =0.02-0.03%
• 2007: 20-30s exposures @ 4x16 positions; =0.01-0.015%
• Overall: 93 nightly measurements for each Stokes parameter
Assumptions
• Lambert sphere approximation
• Rayleigh scattering2 minimization procedure
• Simulated sample of Monte Carlo measurements
Lambert Sphere
• light falling on it is scattered such that the apparent brightness of the surface to an observer is the same regardless of the observer's angle of view
• Perfectly reflecting surface with geometrical albedo p=2/3
Orbital Parameters
• Fixed paramters:orbital period P, transit / periastron epoch T0, semi-major axis a, radius of the star R*
• Free parameters: eccentricity e, inclination i, longitude of the ascending node , radius of Lambert sphere RL, constant shift in Stokes parameters u and q
Inclination
• Inclination can be tested by photometric data but polarimetry can destinguish between i>90° (clockwise roation) and i<90° (counterclockwise)
• At i=0° q and u have the same amplitude• For i≠0°: relative amplitude is influenced by
, variations appear only at certain longitudes
Best-fit Solution
(1) Errors of measurements have Gaussian distribution
(2) Signal is not spurious
(3) Solution is robust to errors
Interpretation
• Excellent agreement between known values in e and i indicate plausibility of Rayleigh scattering
• Planet has extended atmosphere effectively scattering in blue
• Small particles like H, H2, H2O or even small dust grains (≤ 5m, e.g. silicate) might be present
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