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First Detection of Polarized Scattered Light from an Exoplanetary AtmosphereBerdyugina et al. (12/2007) Florian Herzele SE 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
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°)
Stokes Parameter • X-axis: north-south Y-axis: east-west
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 scattering • 2 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'sangle 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 Errors of measurements have Gaussian distribution Signal is not spurious Solution is robust to errors
Results 180° - ≈ < ≈30% if evaporating halo exists
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