POLARIMETRIC OBSERVATIONS OF TNOs AND CENTAURS AT THE ESO-VLT

1. POLARIMETRIC OBSERVATIONS OF TNOs AND CENTAURS AT THE ESO-VLT Irina Belskaya Institute of Astronomy, Kharkiv National University, Ukraine In collaboration with: S. Bagnulo1, A. Stinson1, G.P. Tozzi2, K. Muinonen3, M.A. Barucci4, S. Fornasier4 1Armagh Observatory, UK 2INAF - Oss. Astrofisico di Arcetri, Italy 3University of Helsinki, Finland 4LESIA-Observatoire de Paris. France

2. Specificity OF TNOs POLARIMETRY • Geometry of ground-based observations of distant objects is very limited Main-belt asteroids r 3 AU 19.5 10 AU 5.8 40 AU 1.4 Phase angle range for TNOs • Most objects are very faint and their observations require large telescopes • V~20m σP~0.05% t~2h at VLT

3. FIRST POLARIMETRIC OBSERVATIONS of a TNO AT THE ESO-VLT Plutino (28978) Ixion (D=600 km, pR=0.15) R~ 19.7 mag Pmin ≥ 1.3% σP ~ 0.1% Boehnhardt et al. (2004) • unusually highnegative polarization at small phase angles • rapid changes with the phase angle

4. FIRST PROGRAM OF POLARIMETRY OF TNOs AND CENTAURS • The polarimetric observations of Ixion demonstrated • the capability of the instrument (FORS1 VLT) to provide good-quality observations of faint objects (20 mag); • 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; • availability of complementary information on object’s physical properties. • belonging to different dynamical groups and spectral classes.

5. POLARIMETRIC OBSERVATIONS AT THE VLT • ~100 h of total observing time at VLT in 2004-2011 (service mode), 11 objects; • observations with FORSusing a remotely controlled rotatable half-wave retarder platein front of the Wollaston prism; • measurements of the linear polarization in the BessellR filter; • instrumental polarization was well-controlled (an accuracy of 0.03% in P and 0.2in the position angle θ).

6. BVRPOLARIMETRY of CENTAUR (2060) Chiron • any wavelength dependence of polarization degree exceeding observational errors

7. LIST OF TNOs AND CENTAURS OBSERVED BY POLARIMETRIC TECHNIQUE 14 objects: 4 dwarf planets, 3 Centaurs, 2 classical, 3 resonant, 2 SDOs

8. DIVERSITY IN THE POLARIZATION PHASE BEHAVIORS • presence of the negative polarization, varying from -0.2% to -2.1% • diverse phase angle behavior of polarization degree

9. MEASURED POLARIZATION DEGREE FOR OBJECTS FROM DIFFERENT ORBITAL CLASSES • for most of the observed objects |Pmin|>1% , • Centaurs show a greater diversity in polarimetric properties compared to TNOs.

10. POSITION OF THE POLARIZATION MINIMUM Chiron: min=1.6 deg, Pmin=–1.4%; Pholus: min≥2.3 deg, Pmin=–2.1%; • Chiron shows the smallest phase angle of polarization minimum • Pholus shows the deepest negative polarization branchat small phase angles

11. TWO DISTINCT BEHAVIOURS OF POLARIZATION-PHASE DEPENDENCES FOR LARGE AND SMALL TNOs • the largest objects (Eris, Pluto, Makemake) 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.

12. RELATIONSHIP of polarization degree and albedo TNOs & CENTAURS ASTEROIDS Masiero et al. (2012) • a single measurement of linear polarization of a TNO at the phase angle about 1 deg can provide a distinction between high- and low-albedo surfaces. • exceptional case of Haumea ( pR=0.84±0.20)

13. RELATIONSHIP OF polarization degree and DIAMETER Volatiles retained Volatiles lost Transition phase • Dependence on capability of retaining volatiles? • Large bodies with methane ice rich (Pluto, Eris, Makemake) and water ice rich (Haumea, Quaoar) surfaces show different polarization properties.

14. CONCLUSIONS • The first reconnaissance stage of applying polarimetry to study TNOs seems to be over. • Main observational features can be summarized as following: • - noticeable negative polarization have been measured for all observed distant objects, varying from -0.2% to -2.1%; • polarization minima occur at small phase angles; • two distinct polarization phase behaviours for large and small TNOs; • all three Centaurs observed so far show diverse polarization behaviour; • |Pmin|tends to decrease when surface albedo increases; • different polarization properties of TNOs with methane ice and water ice rich surfaces. • We need to understand future prospects of polarimetry in study of distant objects.