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°)

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

Obtained Data

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

Rayleigh Scattering

2 Distribution

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

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

Planetary Motion

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