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Polarization Phase Angle Dependence POLARIMETRIC PROPERTIES OF TRANSNEPTUNIAN OBJECTS AND CENTAURS Irina Belskaya LESIA, Observatoire de Paris, France Institute of Astronomy, Kharkiv National University, Ukraine POLARIZATION PHASE ANGLE DEPENDENCE Main-belt asteroids Geometry of the ground- based observations of distant objects is very limited: r 3 AU 19.5 10 AU 5.8 40 AU 1.4 Phase angle range for TNOs FIRST MEASUREMENTS OF POLARIZATION FOR A TNO 0.4 Pluto 1972 1979 Kelsey & Fix 1973 1980 0.2 1981 Breger & Cochran 1982 1988 Avramchuk et al. 1992 0.0 V~ 15 mag P, % -0.2 Telescopes: 1 m, 1.3 m, 2 m σP~0.03-0.05% -0.4 V (1979-1981) or without filters -0.6 0.0 0.5 1.0 1.5 2.0 Phase angle, deg - negative polarization - no evidence of polarization variations with rotation - no noticeable phase angle dependence • Pluto’s surface is microscopically rough but not “asteroid-like” • Polarization from a thin atmosphere adds to the surface polarization? NEW ERA IN POLARIMETRIC OBSERVATIONS Plutino 28978 Ixion (2001 KX76) 0.5 Boehnhardt et al. 2004 Ixion 0.0 R~ 19.7 mag -0.5 Telescope: 8 m VLT, FORS1 -1.0 P ≥ 1.3% Polarization degree,% Polarization min -1.5 σP ~ 0.1% -2.0 0.0 0.5 1.0 1.5 2.0 Phase angle, deg - unusually high negative polarization - rapid changes with the phase angle • Two component surface model (mixture of bright and dark scatters) FIRST PROGRAM OF POLARIZATION MEASUREMENTS OF TNO AND CENTAURS The polarimetric observations of Ixion demonstrated (a) the capability of the instrument (FORS1 VLT) to provide good-quality observations of faint objects (20 mag); (b) the capability of the polarimetric technique to study distant objects even if they are observable only at very small phase angles. The aim of new polarimetric observations was to probe surface properties of objects from different dynamical groups . The following criteria were used to select objects : • V≤ 21 mag. It lets to measure a polarization degree with accuracy better than 0.1% in less than two hours telescope time at 8 m telescope; • the possibility to cover the largest phase angle range reachable from ground- based observations; • dynamical group; • availability of complementary information on object’s physical properties. LIST OF OBJECTS OBSERVED BY POLARIMETRIC TECHNIQUE Object Type Group (deg) Reference (2060) Chiron Centaur BB 0.54.2 LP, Bagnulo et al. (2006) (5145) Pholus Centaur RR-U 0.9- 2.6 LP, Belskaya et al. (2010) (10199) Chariklo Centaur BR 2.74.4 LP, Belskaya et al. (2010) (20000) Varuna Classical IR 0.11.3 LP, Bagnulo et al. (2008) Scattered IR (26375) 1999 DE9 0.11.4 LP, Bagnulo et al. (2008) (28978) Ixion Resonant IR-RR 0.21.3 Boehnhardt et al. (2004) Scattered BR 0.83.1 Rousselot et al. (2005) (29981) 1999 TD10 (38628) Huya Resonant IR 0.62.0 LP, Bagnulo et al. (2008) (50000) Quaoar Classical IR-RR 0.21.2 Bagnulo et al. (2006) (134340) Pluto Resonant BR 0.71.8 Breger&Cochran (1982) (136108) Haumea Classical BB 0.99 LP, Bagnulo et al. (2008) (136199) Eris Detashed BB 0.10.6 LP, Belskaya et al. (2008) Most part of the data were obtained as part of ESO-VLT Large Program (LP) in 2006-2008 (178.C-0036, PI: A. Barucci). POLARIMETRY TEAM: I. Belskaya, S. Bagnulo, K. Muinonen, S. Fornasier, G.P. Tozzi, L. Kolokolova POLARIMETRIC OBSERVATIONS WITH VLT • 48 h of total observing time at VLT (service mode); • observations with FORS1 using a remotely controlled rotatable half-wave retarder plate in front of the Wollaston prism; • measurements of the linear polarization in the Bessell R filter; • instrumental polarization was well-controlled (an accuracy of 0.03% in P and 0.2 in the position angle θ). LP-VLT: 8 objects: 3 Centaurs, 2 classical, 2 resonant, 1 detached DIVERSITY IN POLARIZATION PHASE BEHAVIOR 0.0 0.0 0.0 0.0 P, % Quaoar P, % Huya P, % Chiron P, % Pholus -0.5 -0.5 -0.5 -0.5 -1.0 -1.0 -1.0 -1.0 -1.5 -1.5 -1.5 -1.5 -2.0 -2.0 -2.0 -2.0 -2.5 -2.5 -2.5 -2.5 0 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 Phase angle, deg Phase angle, deg Phase angle, deg Phase angle, deg • presence of the negative polarization, varying from -0.3% to -2.3% • diverse phase angle behavior of polarization degree TWO BEHAVIOURS OF POLARIZATION-PHASE DEPENDENCES OF TNOs - the largest objects (Eris, Pluto, Quaoar) show a shallow branch of the polarization-phase curve with slow changes of polarization with the phase angle; - the smaller objects (Huya, Ixion, Varuna, 1999 DE9) show a rapid enhancement in the negative polarization reaching about -1% at the phase angle of 1 deg. Huya Ixion 0.0 0.0 Varuna 1999 DE9 -0.5 -0.5 -1.0 -1.0 Eris -1.5 Pluto -1.5 Polarization degree, % Haumea Polarization degree, % Quaoar -2.0 -2.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 Phase angle, deg Phase angle, deg POSSIBLE REASONS OF TWO POLARIZATION BEHAVIOURS Dependence of polarization degree on albedo? 1.5 , % on capability of retaining volatiles? 99DE9 IxionVaruna Huya 1.0 of phase angle Quaoar Haumea 1 deg 0.5 Pluto Eris 0.0 Polarization at 0.01 0.1 1 Albedo A single measurement of linear polarization of a TNO at the phase angle about 1 deg can Vertical lines indicate the transition between provide a distinction between volatile-free and volatile-rich surfaces high- and low-albedo surfaces. according to Schaller&Brown (2007). COMPARISON WITH OTHER SOLAR SYSTEM BODIES 3 Centaurs Encke TNOs Elst-Pizzaro asteroids Iapetus 2 comets Himalea 1 0 -1 Polarization degree, % degree, Polarization -2 -3 0 5 10 15 20 Phase angle, deg Polarization phase behaviours of TNOs and Centaurs are significantly different from those measured for asteroids and satellites. Data are taken from ADS data-bases and Bagulo et al. (2008, 2010), Boehnhardt et al. (2008), Kiselev et al. (2010). POLARIZATION MINIMUM Chiron: min=1.6 deg, Pmin=–1.4%; Pholus: min≥2.3 deg, Pmin=–2.1%; 0.0 Europa EE E 0.5 V S S S S M S Callisto (D) Chariklo 1.0 MM Enke F Iapetus (D) F MElst-Pizarro Moon M |, % Chiron C P Deimos min C C Halley P 1.5 | C J6 C C CG C C C 2.0 Pholus 2.5 0 2 4 6 8 10 12 14 , deg min • Chiron shows the smallest phase angle of polarization minimum • Pholus shows the deepest negative polarization branch at small phase angles POSSIBLE REASONS OF SHIFT OF POLARIZATION MINIMUM TOWARD SMALL PHASE ANGLES • fluffy surfaces with a large portion of submicron or micron-sized particles; • inhomogeneous surface matter (scatterers with small and large single-scattering albedos). 0.0 P, % compressed full frost 3.2 m -0.5 1 m medium frost 0.1-0.5 m -1.0 fluffy minimum frost Sub-micron particles 0 1 2 3 4 Phase angle (deg) Measurements of size-separated Frost of submicron ice Al2O3 (Shkuratov et al. 2002) Fluffy and compressed crystals on a dark surface MgO powder of ~1 µm (Dougherty & Geake 1994) (Shkuratov et al. 2002) MODELING BASED ON THE COHERENT BACKSCATTERING MECHANISM • effective for high ERIS & PLUTO Eris (Belskaya et al. 2008) albedo surfaces; -1.8 Eris (Sheppard 2007) Eris (Duffard et al. 2007) Pluto (Buratti et al. 2003) • brightness peak is -1.6 accompanied by -1.4 polarization peak of R magnitude -1.2 similar half-width; -1.0 0.0 0.5 1.0 1.5 2.0 • both effects were Phase angle (deg) Mishchenko et al. (1993, 2006) observed for bright satellites. 0.0 -0.5 -1.0 Linear polarization (%) Eris -1.5 Pluto 0.0 0.5 1.0 1.5 2.0 Phase angle (deg) Fit by Muinonen’s model 0 4 8 10 CONCLUSIONS • Polarimetric observations of 8 objects made within the ESO-VLT Large Program increase to 12 the number of TNOs and Centaurs for which polarimetric data are available. Noticeable negative polarization have been measured for all observed distant objects, varying from -0.3% to -2%. A dependence of polarization degree on object’s albedo is well-seen. • All four small TNOs (D<700 km) in our sample have similar polarization- phase dependences in spite of belonging to different dynamical groups. On the contrary, each of three Centaurs measured so far show different polarization-phase behaviour. It suggests a greater diversity of surface texture among Centaurs. • Polarization phase behaviours both TNOs and Centaurs are significantly different from those measured for asteroids. It gives an evidence of different surface texture of these bodies. .
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