Transcript First Detection of Polarized Scattered Light from an Exoplanetary

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 HD189733
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 (± 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 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 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 (≤ 5m, e.g. silicate)
might be present
Planetary Motion
QuickTi me™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.