Polarization Measurements of TNOs and Centaurs

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)

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3. ... 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 (~500-1000) • 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 P @ 1 deg phase in solar system • Chiron: minimum P @ 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 coma @ >29mag/sarcsec in summer 2004 More information: Boehnhardt et al., 2004, A&A 415, L21 Bagnulo et al., 2006, A&A 450, 1239

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