Polarimetry of gas planets

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Polarimetry of gas planets

• Franco Joos
• 21.06.05

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Contents

• Motivation
• Polarization in planetary atmospheres
  limb polarizatoin
• Polarization observations on Uranus and
  Neptune a. Imaging polarimetry b.
  Spectropolarimetry
• Summary conclusions

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1. Motivation

• search for extra-solar planets and investigation
  of atmospheric structure of our planets
• for the detection of extra-solar planets we
  concentrate on direct imaging
• bad contrast but interesting expectations
• --gt go for polarimetry

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1. Motivation

• hard to predict the amount of polarization in
  extra-solar planets
• only solar sytem planets are available up to now
• measurements from space (pioneer) and from
  Earth(us)
• backscattering small phase angles for solar
  system giant gas planets ( 3 for
  Uranus)--gtonly little polarization

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2. Polarization in planetary atmospheres limb
polarization

• known second-order scattering effect producing
  net linear limb polarization perpendicular to the
  planet's limb
• -- gt extrapolating from small phaseangles to
  larger ones to predict the polarization for
  extra-solar planets, which are expected to have
  phaseangles between 60 to 120 degrees

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2. Polarization in planetary atmospheres limb
polarization
1st order

• situtation of grazing incidence at the limb
• single scattering (backscattering) produces
  no net polarization

• favours polariztion perpendicular to the limb

• photons going up or down after first scattering
• have a parallel polarization
• up high probability of escape without further
• scattering
• down high probability of absorption or multiple
• scatterings

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3. Polarization observations on Uranus and Neptune

• EFOSC2 Observations at the 3.6m telescope at La
  Silla, 29th and 30th of November 2003
• presentation of Uranus and Neptune data
• Imaging and spectropolarimetry

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3a. Imaging polarimetry

• diameter 3.51''
• phase angle 2.8
• imagescale 0.157''/pixel
• seeing 0.8''

N
1''
W
Uranus intensity image in i-band
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3a. Imaging polarimetry
Uranus polarized flux in Stokes-Q direction
(left) and Stokes-U (right) in the i-band. Q I0
I90 and U I45 I135
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3a. Imaging polarimetry

• defining radial Stokes parameters Qr and Ur
• Qr Q cos(2?) U sin(2?) where ? is the polar
  angle with respect to North
• ? arctan(x-x0)/(y-y0)
• Qr gt 0 indicates a polari-zation perpendicular
  to the limb

N
E
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3a. Imaging polarimetry
Uranus radial Stokes Qr in the i-band
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3a. Imaging polarimetry

• diameter 2.24''
• phase angle 1.7
• imagescale 0.157''/pixel
• seeing 0.8''

Neptune intensity image in i-band
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3a. Imaging polarimetry
Neptune polarized flux in Stokes-Q direction
(left) and Stokes-U (right) in the R-band.
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3a. Imaging polarimetry
Neptune radial Stokes Qr in the R-band
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3b. Spectropolarimetry

• spectropolarimetric observations of Uranus and
  Neptune (5300 9300 Å)
• slitwidth of 0.5'' (Uranus) and 1.5'' (Neptune)
• slit direction N-S for both and E-W for Uranus
• spacially well resolved Uranus, less for Neptune

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3b. Spectropolarimetry
Uranus N-S slit position, white is the central
part and black squares are the limb parts
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3b. Spectropolarimetry
Uranus N-S solid line limb of the planet dotted
line central part dashed line average
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3b. Spectropolarimetry
Uranus N-S solid line limb of the planet dotted
line central part dashed line average
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3b. Spectropolarimetry
Uranus N-S solid line limb of the planet dotted
line central part dashed line average
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3b. Spectropolarimetry
Uranusscan in N-S in the methane- absorption band
at 6190 (10) Å 2.5 to 3.5 Q/I at the limbs
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3b. Spectropolarimetry
Selection of the 'conti- nuum' from 6300 to 6400
Å
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3b. Spectropolarimetry
Uranusscan in N-S in the 'continuum' from 6300 -
6400 Å 2 Q/I at the limbs
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3b. Spectropolarimetry
Neptune N-S slit position, white is the central
part and black squares are the limb parts 1.5''
wide slit -gt nearly entire planet covered
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3b. Spectropolarimetry
Neptune N-S spectrum solid line limb of the
planet dotted line central part dashed line
overall average
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4. Summary conclusions

• the limbs of Uranus and Neptune show higher
  polarization than the central region due to the
  second-order scattering effect
• extrapolating these polarization values to 90
  phase angle gives us more than 25 disc
  integrated polarization.
• can be used for search and characterization of
  extra-solar planets
• enhanced polarization in the methane absorption
  bands compared to the adjacent continuum
• probably high lying methane layers in the planets
  atmospheres preventing multiple scattering

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4. Summary conclusions

• decrease of polarization from blue to red
  (typical for Rayleigh scattering)
• extra-solar planets will (probably strongly) be
  polarized if they have similar atmospheres like
  our outer giant gas planets.

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THE END
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z-filter
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R-filter
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i-filter
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Uranusscan in N-S in the red part from 8000 -
9000 Å 1.5 Q/I at the limbs
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3b. Spectropolarimetry
Uranus E-W slit position white is the central
part and black squares are the limb parts
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3b. Spectropolarimetry
Uranus E-W solid line limb of the planet dotted
line central part dashed line average
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3b. Spectropolarimetry
Uranusscan in E-W in the methane- absorption band
at 6190 (10) Å 2 to 3 Q/I at the limbs
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3b. Spectropolarimetry
Uranusscan in E-W in the 'continuum' from 6300 -
6400 Å 1.5 Q/I at the eastern limb and up to 3
at the western limb
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Uranusscan in E-W in the 'continuum' from 8000 -
9000 Å 1 - 1.5 Q/I at the limbs