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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
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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
• V20m sP0.05 t2h at VLT

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FIRST POLARIMETRIC OBSERVATIONS of a TNO AT THE
ESO-VLT
Plutino (28978) Ixion (D600 km, pR0.15)
R 19.7 mag Pmin 1.3 sP 0.1
Boehnhardt et al. (2004)

• unusually high negative polarization at small
  phase angles
• rapid changes with the phase angle

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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
  objects physical properties.
• belonging to different dynamical groups and
  spectral classes.

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POLARIMETRIC OBSERVATIONS AT THE VLT

• 100 h of total observing time at VLT in
  2004-2011 (service mode), 11 objects
• observations with FORS 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 ?).

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BVR POLARIMETRY of CENTAUR (2060) Chiron

• any wavelength dependence of polarization degree
  exceeding observational errors

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LIST OF TNOs AND CENTAURS OBSERVED BY
POLARIMETRIC TECHNIQUE
Object Type Tax D (km) ? (deg) Reference
(2060) Chiron Centaur BB 233 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 259 2.7?4.4 Belskaya et al. (2010)
(20000) Varuna Classical IR 500 0.1?1.3 Bagnulo et al. (2008)
(26375) 1999 DE9 Scattered IR 461 0.1?1.4 Bagnulo et al. (2008)
(28978) Ixion Resonant IR 590 0.2?1.3 Boehnhardt et al. (2004)
(29981) 1999 TD10 Scattered BR 104 0.8?3.1 Rousselot et al. (2005)
(38628) Huya Resonant IR 439 0.6?2.0 Bagnulo et al. (2008)
(50000) Quaoar Classical RR 1109 0.2?1.2 Bagnulo et al. (2006)
(90482) Orcus Resonant BB 956 1.1 Belskaya et al. (2012)
(134340) Pluto Resonant BR 2350 0.7?1.8 BregerCochran (1982)
(136108) Haumea Classical BB 1500 1.0 Bagnulo et al. (2008)
(136199) Eris Detashed BB 2400 0.1?0.6 Belskaya et al. (2008)
(136472) Makemake Classical BR 1420 0.6-1.1 Belskaya et al. (2012)
14 objects 4 dwarf planets, 3 Centaurs, 2
classical, 3 resonant, 2 SDOs
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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

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MEASURED POLARIZATION DEGREE FOR OBJECTS FROM
DIFFERENT ORBITAL CLASSES

• for most of the observed objects Pmingt1 ,
• Centaurs show a greater diversity in
  polarimetric properties compared to TNOs.

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POSITION OF THE POLARIZATION MINIMUM
Chiron ?min1.6 deg, Pmin1.4 Pholus
?min2.3 deg, Pmin2.1

• Chiron shows the smallest phase angle of
  polarization minimum
• Pholus shows the deepest negative polarization
  branch at small phase angles

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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.

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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 ( pR0.840.20)

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RELATIONSHIP OF polarization degree and DIAMETER
Volatiles retained
Transition phase
Volatiles lost

• 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.

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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.