First Detection of Polarized Scattered Light from an

First Detection of Polarized Scattered Light from an Exoplanetary Atmosphere Berdyugina 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

System HD 189733 Name 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 (± 2 e-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 60 cm 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 -15 s exposures @ 2 x 16 positions; =0. 02 -0. 03% • 2007: 20 -30 s exposures @ 4 x 16 positions; =0. 01 -0. 015% • Overall: 93 nightly measurements for each Stokes parameter

Obtained Data

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'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 T 0, 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, H 2 O or even small dust grains (≤ 5 m, e. g. silicate) might be present

Planetary Motion