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Home , 28978 Ixion

OF TRANSNEPTUNIAN OBJECTS AND

Irina Belskaya Institute of Astronomy, Kharkiv National University, Ukraine & Stefano Bagnulo Armagh Observatory, UK

PHASE ANGLE DEPENDENCE OF POLARIZATION

8 Pmax

6

4

P(α, λ) is determined by the physical P,% 2 properties of the topmost surface layer.

0 inv Phase angle range for TNOs -2 Pmin

0 20 40 60 80 100 120 140 160 Phase angle, deg FIRST POLARIMETRIC OBSERVATIONS of a TNO WITH FORS1 AT THE ESO-VLT

Plutino (28978) (D=600 km, pR=0.15)

0.5 Ixion

0.0 R~ 19.7 mag

-0.5 Pmin ≥ 1.3%

-1.0 σP ~ 0.1%

Polarization degree, % degree, Polarization -1.5 Boehnhardt et al. (2004) -2.0 0.0 0.5 1.0 1.5 2.0 Phase angle, deg • unusually high negative polarization at small phase angles • rapid changes with the phase angle FIRST PROGRAM OF POLARIMETRY 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 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 telescope time at 8 m telescope; • the possibility to cover the largest phase angle range reachable from ground- based observations; • availability of complementary information on object’s physical properties. • belonging to different dynamical group and spectral classes. POLARIMETRIC OBSERVATIONS OF TNOs AT THE VLT

• ~100 h of total observing time at VLT in 2004- 2011 (service mode);

• observations with FORS using a remotely controlled rotatable half-wave retarder plate in front of the Wollaston prism;

• measurements of the linear polarization mainly in the Bessell R filter;

• well-controlled instrumental polarization (an accuracy of 0.03% in P and 0.2 in the position angle θ).

LIST OF TNOs AND CENTAURS OBSERVED BY POLARIMETRIC TECHNIQUE Object Type Tax D (km)  (deg) Reference (2060) Chiron BB 218 0.54.2 Bagnulo et al. (2006)

(5145) Pholus Centaur RR 140 0.9- 2.6 Belskaya et al. (2010) (10199) Chariklo Centaur BR 248 2.74.4 Belskaya et al. (2010) (20000) Varuna Classical IR 668 0.11.3 Bagnulo et al. (2008) Scattered IR (26375) 1999 DE9 311 0.11.4 Bagnulo et al. (2008) (28978) Ixion Resonant IR 617 0.21.3 Boehnhardt et al. (2004) Scattered BR 104 0.83.1 Rousselot et al. (2005) (29981) 1999 TD10 (38628) Huya Resonant IR 458 0.62.0 Bagnulo et al. (2008) (50000) Quaoar Classical RR 1074 0.21.2 Bagnulo et al. (2006) (90482) Orcus Resonant BB 958 1.1 Belskaya et al. (2012) (134340) Resonant BR 2350 0.71.8 Breger&Cochran (1982) (136108) Classical BB 1240 1.0 Bagnulo et al. (2008) (136199) Detached BB 2400 0.10.6 Belskaya et al. (2008) (136472) Classical BR 1420 0.6-1.1 Belskaya et al. (2012)

14 objects (4 dwarf ): 3 Centaurs, 4 classical, 4 resonant, 3 SDOs DIVERSITY OF 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 -0.2% to -2.3%

• diverse phase angle behavior of polarization degree

TWO DISTINCT BEHAVIOURS OF POLARIZATION-PHASE DEPENDENCES FOR LARGE AND SMALL TNOs • the largest objects show a shallow branch of the polarization-phase curve with slow changes of polarization with the phase angle; • the smaller size objects show a rapid enhancement in the negative polarization reaching about -1% at the phase angle of 1 deg. (Bagnulo et al. A&A 491, L33-36, 2008)

0.0 0.0

-0.5 -0.5

-1.0 -1.0

, % ,

, % , P P Eris -1.5 Pluto -1.5 Huya Haumea Ixion Quaoar Varuna 1999 DE9 -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

POLARIMETRY OF PLUTO

0.4 Pluto 1972 1979 1980 Kelsey & Fix 1973 0.2 1981 1988 Breger & Cochran 1982 Avramchuk et al. 1992

0.0

P, % V~ 15 mag -0.2 Telescopes: 1 m, 1.3 m, 2 m

-0.4 σP~0.03-0.05% 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 -- measurements refer to the Pluto- system -0.35% ≤ P(Pluto) ≤ -0.1% if -1.5% ≤ P(Charon) ≤ 0% POLARIZATION-PHASE ANGLE DEPENDENCE OF THE LARGEST TNOs

0.0

-0.5

-1.0

, % , P Eris -1.5 Pluto Haumea Quaoar -2.0

0.0 0.5 1.0 1.5 2.0 Phase angle, deg POLARIZATION-PHASE ANGLE DEPENDENCE OF THE LARGEST TNOs

0.0

-0.5

-1.0

, % , P Eris -1.5 Pluto Haumea Quaoar -2.0 Makemake

0.0 0.5 1.0 1.5 2.0 Phase angle, deg POLARIZATION-PHASE ANGLE DEPENDENCE OF THE LARGEST TNOs

0.0

-0.5

-1.0

Orcus , % , P Eris -1.5 Pluto Haumea Quaoar -2.0 Makemake

0.0 0.5 1.0 1.5 2.0 Phase angle, deg POLARIZATION-PHASE ANGLE DEPENDENCE OF THE SMALLER SIZE TNOs

0.0

-0.5 Orcus

-1.0

, % ,

P

-1.5 Huya Ixion Varuna 1999 DE9 -2.0 Orcus

0.0 0.5 1.0 1.5 2.0 Phase angle, deg POLARIZATION OF SURFACES OF THE LARGEST TNOs - rich surfaces Water-ice rich surfaces

P , % P , % r r 0.0 0.0

-0.5 -0.5

-1.0 -1.0 Eris Pluto Haumea Makemake Quaoar -1.5 -1.5 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

Barucci et al. (2010) POLARIZATION DEGREE & DIAMETER

2.5

Pholus 2.0

Huya

1.5 ChironDE9 TD10 Ixion

Varuna Orcus

1.0 Chariklo

P|, % | Quaoar Haumea 0.5

Pluto Makemake Eris 0.0

0 500 1000 1500 2000 2500 D, km

Vertical lines indicate the transition between volatile-free and volatile- rich surfaces according to Schaller & Brown (2007) CENTAURS

0.0

-0.5

-1.0

P, % P, -1.5 Chiron Chariklo -2.0 Pholus 1999 TD10 -2.5 0 1 2 3 4 5 6 7 Phase angle, deg

• a great diversity in polarization properties with the negative polarization varying from -1% to -2%. SPECTRAL DEPENDENCE OF POLARIZARION DEGREE

Chiron 0.0

-0.5

-1.0 P, % P,

2004 (B) 2004 (V) -1.5 2004 (R) 2007 (R) 2008 (R)

-2.0 0 1 2 3 4 5 6 7 8 9 Phase angle, deg

• a possible trend for deeper negative polarization in the B band POSITION OF THE POLARIZATION MINIMUM

Chiron: Pmin= –1.4%; min = 1.6º Pholus: Pmin=–2.1%; min ≥2.3º POSITION OF THE MINIMUM OF NEGATIVE POLARIZATION BRANCH 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 ).

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 0 1 2 3 4 Measurements of size- Phase angle (deg) Frost of submicron ice separated 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) POLARIZATION DEGREE &

log(pV) =c1*log(Pmin)+c2 found for asteroids

1 ErisMakemake Haumea Pluto

Orcus

Albedo Quaoar DE9 Ixion Chiron Varuna Huya 0.1 Pholus

Chariklo TD10

0.1 1 P, % • the surface albedo does not play a dominant role in determining the polarization-phase behaviour of distant bodies. • the interplay of the different physical parameters of the surfaces may destroy the correlation with albedo SUMMARY

• Main observational features can be summarized as following: - negative polarization have been measured for all observed objects, varying from -0.2% to -2.1%; - two distinct polarization phase behaviours for large and small TNOs; - different polarization properties of TNOs with methane ice and water ice rich surfaces; - a deep negative polarization with a minimum shifted to small phase angles for small TNOs and Centaurs; - diverse polarization behaviour among Centaurs with a minimum varying from -1% to -2.1% (albedo’s range is 4-16%);

- |Pmin| tends to decrease when surface albedo increases while the correlation weaker as compared to asteroids;

• Interpretation is in progress...