POLARIMETRIC OBSERVATIONS OF TNOs AND CENTAURS

AT THE ESO-VLT

Institute of Astronomy, Kharkiv National University, Ukraine

Irina Belskaya

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

SPECIFICITY OF TNOs POLARIMETRY

• Geometry of ground-based observations of distant objects is very limited

r 3 AU 19.510 AU 5.840 AU 1.4

Phase angle range for TNOs

Main-belt asteroids

• Most objects are very faint and their observations require large telescopes V~20m σP~0.05% t~2h at VLT

FIRST POLARIMETRIC OBSERVATIONS of a TNO AT THE ESO-VLT

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

Boehnhardt et al. (2004)

R~ 19.7 mag

Pmin ≥ 1.3%

σP ~ 0.1%

0.0 0.5 1.0 1.5 2.0-2.0

-1.5

-1.0

-0.5

0.0

0.5

Ixion

Po

lari

zatio

n d

eg

ree

, %

Phase angle, deg

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

FIRST PROGRAM OF POLARIMETRY OF TNOs 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;• availability of complementary information on object’s physical properties.• belonging to different dynamical groups and spectral classes.

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 θ).

BVR POLARIMETRY OF CENTAUR (2060) CHIRON

• any wavelength dependence of polarization degree exceeding observational errors

Object Type Tax D (km) (deg) Reference

(2060) Chiron Centaur BB 233 0.54.2 Bagnulo et al. (2006)

(5145) Pholus Centaur RR 140 0.9- 2.6 Belskaya et al. (2010)

(10199) Chariklo Centaur BR 259 2.74.4 Belskaya et al. (2010)

(20000) Varuna Classical IR 500 0.11.3 Bagnulo et al. (2008)

(26375) 1999 DE9 Scattered IR 461 0.11.4 Bagnulo et al. (2008)

(28978) Ixion Resonant IR 590 0.21.3 Boehnhardt et al. (2004)

(29981) 1999 TD10 Scattered BR 104 0.83.1 Rousselot et al. (2005)

(38628) Huya Resonant IR 439 0.62.0 Bagnulo et al. (2008)

(50000) Quaoar Classical RR 1109 0.21.2 Bagnulo et al. (2006)

(90482) Orcus Resonant BB 956 1.1 Belskaya et al. (2012)

(134340) Pluto Resonant BR 2350 0.71.8 Breger&Cochran (1982)

(136108) Haumea Classical BB 1500 1.0 Bagnulo et al. (2008)

(136199) Eris Detashed BB 2400 0.10.6 Belskaya et al. (2008)

(136472) Makemake Classical BR 1420 0.6-1.1 Belskaya et al. (2012)

LIST OF TNOs AND CENTAURS OBSERVED BY POLARIMETRIC TECHNIQUE

14 objects: 4 dwarf planets, 3 Centaurs, 2 classical, 3 resonant, 2 SDOs

DIVERSITY IN THE POLARIZATION PHASE BEHAVIORS

0 1 2 3-2.5

-2.0

-1.5

-1.0

-0.5

0.0P, %

Chiron

Phase angle, deg0 1 2 3

-2.5

-2.0

-1.5

-1.0

-0.5

0.0P, %

Pholus

Phase angle, deg0 1 2 3

-2.5

-2.0

-1.5

-1.0

-0.5

0.0P, %

Quaoar

Phase angle, deg0 1 2 3

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

Phase angle, deg

P, %

Huya

• presence of the negative polarization, varying from -0.2% to -2.1%

• diverse phase angle behavior of polarization degree

0.0 0.5 1.0 1.5 2.0 2.5

|Pmin

| (%)

CENTAURS

RESONANT

CLASSICAL

SDO

DWARF PLANETS

0 1 2 3 4 5

-2.0

-1.5

-1.0

-0.5

0.0

Chiron

Pholus

Chariklo

Varuna

DE9Ixion TD10

Huya

Quaoar

Orcus

Pluto

Haumea

ErisMakemake

|P|,

%

Phase angle, deg

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.

POSITION OF THE POLARIZATION MINIMUM

Chiron: min=1.6 deg, Pmin=–1.4%;

Pholus: min≥2.3 deg, Pmin=–2.1%;

0.0

0.5

1.0

1.5

2.0

2.50 2 4 6 8 10 12 14

HalleyDeimos

Moon

Chiron

Pholus

CharikloCallisto (D)

Europa

Elst-Pizarro

G

S

V

SSS

M

C

S

C

C

E

C

M

E

C

C

E

F F

CC C

M

MM

P

C

Iapetus (D)

J6

min

, deg

|Pm

in|,

%

Enke

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

• 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.

TWO DISTINCT BEHAVIOURS OF POLARIZATION-PHASE DEPENDENCES FOR LARGE AND SMALL TNOs

0.0 0.5 1.0 1.5 2.0

-2.0

-1.5

-1.0

-0.5

0.0 Huya Ixion Varuna 1999 DE9 Orcus

P,

%

Phase angle, deg

0.0 0.5 1.0 1.5 2.0

-2.0

-1.5

-1.0

-0.5

0.0

Eris Pluto Haumea Quaoar Makemake

P,

%

Phase angle, deg

RELATIONSHIP OF POLARIZATION DEGREE AND ALBEDO

• 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)

0.01 0.1 1

0.1

1

Chiron

Pholus

CharikloVaruna

1999DE9Ixion1999TD10Huya

Quaoar

Orcus

Pluto

Haumea

Eris

Makemake

Pol

ariz

atio

n de

gree

(%

)

Albedo

Masiero et al. (2012)

ASTEROIDS

TNOs & CENTAURS

RELATIONSHIP OF POLARIZATION DEGREE AND DIAMETER

• 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.

0 500 1000 1500 2000 2500

0.0

0.5

1.0

1.5

2.0

2.5

Chiron

Pholus

CharikloVaruna

DE9IxionTD10

Huya

Quaoar

Orcus

Pluto

Haumea

ErisMakemake

|P|,

%

D, km

Transition phaseVolatiles lost Volatiles retained

• 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.

CONCLUSIONS