1. 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)

2. The time-to-leave-the-room-slide
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The time-to-leave-the-room-slide

3. The good, the bad, and the ugly
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The good, the bad, and the ugly

4. 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

5. 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 (~ )
TNOs/Centaurs: low phase angles < 1.5/5deg

6. Object Selection & Observations
Object selection criteria
(relatively) bright (V<21mag)  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)

7. Stokes parameters of linear polarimetry
Opposition surge

8. 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 1 deg phase in solar
system
Chiron: minimum 1.5 – 1.8 deg
Chiron: P independent of filter / wavelength

9. 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

10. Results III (Modeling)
Object specific (cum grano salis)
all 3 objects: free path length of dark component >> wavelength
Chiron: dito for bright component
dark component in Ixion&Quaoar and bright component in Chiron resemble one another
 observations & modelling:
Ixion & Chiron may have similar surface structure
Quaoar is significantly different (darker)

11. 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 >29mag/sarcsec in summer 2004
More information: Boehnhardt et al., 2004, A&A 415, L21
Bagnulo et al., 2006, A&A 450, 1239

12. To the audience: Thank you for your patience
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To the audience: Thank you for your patience
To thoses who went outside: I hope you had a good time