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Polarimetric direct detection ofextra-solar Planets with
SPHERE/ZIMPOL
“In the spirit of Bernard Lyot”Berkeley, 4.-8. June 2007
Franco JoosETH Zürich, Switzerland
● A lot of different approaches to directly detectextra-solar planets. We will do it with
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● A lot of different approaches to directly detectextra-solar planets. We will do it with
SPHERE/ZIMPOL
● SPHERE is a second generation ESO-VLTinstrument with three complementary focalplane instruments aiming to directly detectand analyze giant extra-solar planets
● Two near-IR instruments searching for self-luminous young planets. The third is a high-precision imaging polarimeter ZIMPOLdetecting and analyzing reflected light fromolder planets
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● There will be an overview talk on SPHEREby Jean-Luc Beuzit (PI) tomorrow
● Poster by Anthony Boccaletti: developmentof coronagraphs for SPHERE
● I will concentrate on the polarimetricinstrument ZIMPOL which is the Zürichcontribution to SPHERE
● One of the main problem if trying to directlydetect an extra-solar planets from theground:
The planet is MUCH fainter than the halo ofthe stellar PSF
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● One of the main problem if trying to directlydetect an extra-solar planets from theground:
The planet is MUCH fainter than the halo ofthe stellar PSF
Solution: Use extreme AO and coronagraphyand go for differential technique, searchingfor a signal present at the planet position butnot present in the stellar PSF
● One of the main problem if trying to directlydetect an extra-solar planets from theground:
The planet is MUCH fainter than the halo ofthe stellar PSF
Solution: Use extreme AO and coronagraphyand go for differential technique, searchingfor a signal present at the planet position butnot present in the stellar PSF
imaging polarimetry
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● Light reflected by a planet can highly bepolarized, whereas the starlight can beassumed to be unpolarized
● Light reflected by a planet can highly bepolarized, whereas the starlight can beassumed to be unpolarized
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● A polarimetric measurement is obtained bytaking the difference of the two intensities atorthogonal polarization directions ( I∥ - I⊥).The normalized difference gives thepolarization degree: p = Q/I = (I∥ - I⊥)/(I∥ + I⊥).
● ZIMPOL (Zürich IMaging POLarimeter) is ahigh-precision imaging polarimeter providinga polarimetric precision of better than 10-5
working in the range of 600 to 900nm forSPHERE
synchronization (kHz)
modulator
polarizer
demodulatingCCD detector
S(t) I(t)Spolarization
signalmodulatedpolarization
signal
modulatedintensitysignal
ZIMPOL measuring principle:
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● After thousands of modulation cycles theCCD is read out in less than a second
● ZIMPOL image contains two sub-images ofthe same object at orthogonal polarizationdirections
● Compute the difference and normalize → Q/I
ZIMPOL polarizationmeasurements at 630nmfor Jupiter and Saturn atMc Math Pierce telescope,Kitt Peak
I
Q/I
U/I
Example: Jupiter andSaturn
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Summary of the ZIMPOL technique:
• images of two opposite polarization modes arecreated simultaneously
→ modulation faster than seeingvariations• both images are recorded with same pixel
→ both images are subject to the sameaberrations (chromatic effects)• integration over many modulation cycleswithout readout (low RON)
Precision of 10-5 is routinely achieved in solarapplications
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THE END
Noise CurveNoise Curve
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ZIMPOL camera (1. generation)
limb
total slit
center
Example of “long-slit” spectropolarimetry of Uranus
Joos & Schmid, 2007, A&A
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polarization p(90) vs. reflectivityf(90)
Solar system planets surfaceproperties
p(90) f(90)rockyMercury 5-10% lowMars 5-10% low
cloudy (little Rayleigh scatt.)Venus <5% (–) highSaturn <5% high
cloudy and Rayleigh scatt.Jupiter 5-20% highEarth 5-20% high
strong Rayleigh scatteringUranus >15% med.Neptune >15% med.Titan 50% med.
R-band
● The orientation of theexo-system matters,because thepolarization dependson the phase angle:– always perfect for a
face-on system– twice a year perfect
for an inclined system
star
planet
polarization
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Intensity ratio of exo-Sun and exo-Jupiterat 1'' separation ≈108.
108
logI
1''
Example: Jupiter-Sun system at 5pc
Intensity ratio of exo-Sun and exo-Jupiterat 1'' separation ≈108.
Degradation of the point-like sources duetothe atmosphere→ bright halo of the star→ overlapping of the two intensities→ contrast at the position of the planet≈107
→ too large for an imaging polarimeter
108
logI
107
logI
1''
Example: Jupiter-Sun system at 5pc
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Intensity ratio of exo-Sun and exo-Jupiterat 1'' separation ≈108.
Reducing the contrast by extreme AOand coronagraphy to ≈104. Contrast inthe regime of the most sensitiveimagingpolarimeters.
Degradation of the point-like sources duetothe atmosphere→ bright halo of the star→ overlapping of the two intensities→ contrast at the position of the planet≈107
→ too large for an imaging polarimeter
107
logI
108
logI
104
logI
1''
Example: Jupiter-Sun system at 5pc
Intensity ratio of exo-Sun and exo-Jupiterat 1'' separation ≈108.
Reducing the contrast by extreme AOand coronagraphy to ≈104. Contrast inthe regime of the most sensitiveimagingpolarimeters.
Degradation of the point-like sources duetothe atmosphere→ bright halo of the star→ overlapping of the two intensities→ contrast at the position of the planet≈107
→ too large for an imaging polarimeter
107
logI
108
logI
104
logI
1''
Example: Jupiter-Sun system at 5pc
ZIMPOL
