Polarization Measurements of TNOs and Centaurs

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Polarization Measurements of TNOs and Centaurs

• Stefano Bagnulo (ESO Santiago de Chile)
• Antonella Barucci (Obs. Paris-Meudon)
• Irina Belskaya (Univ. Kharkiv)
• Hermann Boehnhardt (MPS Katlenburg-Lindau)
• Ludmilla Kolokolova (Univ. Maryland)
• Karri Muinonen (Univ. Helsinki)

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• The time-to-leave-the-room-slide
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• The good, the bad, and the ugly
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The Physical Link

• Light scattering of minor (atmosphereless) bodies
  like TNOs - in the solar system allows to
  measure
• 1. global albedo size
• ? photometry (vis. submm)
• 2. composition of surface
• ? spectroscopy (vis. IR)
• 3. micro-roughness grain albedo
• ? photometry polarimetry (vis.)
• ? physical interpretations of measurements
  mentioned in items 1-3 are intimately linked and
  require consistent/complementary modeling
  approach

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Polarization Opposition Surge

• Light reflection at surface produces
• linear polarization (phase angle dependant)
• opposition surge (low phase angle only)
• Contributing physical effects
• shadowing effect
• coherent back-scattering
• ? properties of surface grains on microscopic
  scale
• proper modeling approach
• Some physical constraints
• polarimetry ? lots of SNR (500-1000)
• TNOs/Centaurs low phase angles lt 1.5/5deg

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Object Selection Observations

• Object selection criteria
• (relatively) bright (Vlt21mag) ? SNR (? big
  objects)
• good coverage of phase angle range ? phase curve
• albedo spectrum measured ? model constraints
• Telescope instrument
• VLT 8.2m FORS1 (in service mode 1-2h every
  1-3 weeks)
• linear polarimetry in broadband filters R
  filter photometry
• (quasi-simultaneous)
• Objects observed (2 campaigns)
• 28798 Ixion (Plutino) R phot. R lin.pol.
  (0.2 1.35 deg)
• 2060 Chiron (Centaur) R phot. BVR lin.pol.
  (1.4 4.2 deg)
• (additional goal wavelength dependence of
  lin.pol.)
• 50000 Quaoar (CDO) R phot. R lin.pol. (0.25
  1.25 deg)

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Stokes parameters of linear polarimetry
Opposition surge
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Results I (Observations)

• Opposition surge
• slope 0.04 0.12 mag/deg
• ? within range of other solar system
• bodies
• variability within lightcurve amplitude

• Polarization
• negative Stokes P
• ? largest P _at_ 1 deg phase in solar
• system
• Chiron minimum P _at_ 1.5 1.8 deg
• Chiron P independent of filter / wavelength

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Results II (Modeling)

• Model spherical Rayleigh scatterers
  characterized by grain albedo and free path
  length, multiple scattering, multipole expansion
  (Muinonen, 2004)
• General conclusions
• single grain type model does not fit the
  measurements (applies for all 3 objects)
• two-component (bright/dark) mixture with
  different mean free path
• ? 5 parameter model grain albedo free path
  length (dark/bright), mixing ratio
• 2-component model fits the data and produces
  parameters that are consistent with complementary
  results on global albedo and spectral modeling of
  surface reflectivity

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Results III (Modeling)

• Object specific (cum grano salis)
• all 3 objects free path length of dark component
  gtgt wavelength
• Chiron dito for bright component
• dark component in IxionQuaoar and bright
  component in Chiron resemble one another
• ? observations modelling
• Ixion Chiron may have similar surface structure
• Quaoar is significantly different (darker)

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Concluding Slide

• Polarimetry possible useful tool for surface
  characterization of TNOs and Centaurs (careful
  target selection!)
• TNO Centaurs polarimetric properties seem to
  be distinctly different from other small bodies
  around the Sun
• Sample neither trend nor statistical analysis of
  group properties possible so far
• Modeling things are happening

• By-product 2060 Chiron no coma _at_ gt29mag/sarcsec
  in summer 2004

More information Boehnhardt et al., 2004, AA
415, L21 Bagnulo et al., 2006, AA 450,
1239
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• To the audience Thank you for your patience
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• To thoses who went outside I hope you had a good
  time
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