2017 East Asian Young Astronomers Meeting @Ishigakijima, JP

UT 2017 NOV 14 (TUE)

Optical and Near-Infrared
Polarimetry in
Cometary Sciences
Y. G. Kwon1, M. Ishiguro1, Y. Shinnaka2, T. Nakaoka3, K. S. Kawabata3, H.
Hanayama4, D. Kuroda5, M. Kawabata3, M. Uemura3, and H. Akitaya6
1 Seoul National University, Republic of Korea
2 National Astronomical Observatory of Japan
3 Hiroshima University, Japan
4 Ishigakijima Astronomical Observatory, Japan
5 Okayama Astrophysical Observatory, Japan
6 Ibaraki University, Japan

Image credit: ESA NavCam/Rosetta

Table of Contents
n Introductionn Observation
q OISTER (Optical and Infrared Synergetic Telescopes for

Education and Research) Networkn Results & Discussion
q Unimodal distribution of polarization phase function ofcometary dust particles
q Relationship between Pmax – TJ : Implication on Size of dustn Summary1. Introduction

Why Cometary Dust?
Comets are the relics of the formation
epoch of the solar system.

http://www.lunartech.org/research/
http://news.softpedia.com/news

astronomy.swin.edu.au

They start the orbital motions around the Sun and approach the
Earth, ejecting such ancient materials.
We can have the opportunity to decipher the primitive information
which have buried underneath the cometary surface for a long time!

https://ase.tufts.edu/cosmos/view_picture.asp?id=619

https://apod.nasa.gov/apod/ap150121.html

http://blogs.esa.int/rosetta/2015/03/04/all-round-activity-cometwatch-25-26-27-february/

1. Introduction

Polarimetry of Cometary Dust Particles
Iㅗ
Scattering PlaneI//

α: Phase Angle

Scattered sunlight reflects
the physical characteristics
of a single scatter in
diffuse media likecometary coma.
(Rosenbush, et al. 2007)

From the observed patterns of scattered light,
we can constrain the physical properties of cometary dust, such assize, porosity, and albedo.

2. Observations

OISTER
Optical and Infrared Synergetic Telescopes for Education and Research
SNU

Nayoro

Kiso
HHO

NHAO
OAO

IAO
NAOJ
IRSF

2. Observations

OISTER
Optical and Infrared Synergetic Telescopes for Education and Research

HONIR (Hiroshima Optical and Near-IR Camera)n Optical and NIR simultaneous spectroscopy and polarimetryn Comet C/2013 US10 (Catalina): UT 2015-12-17 – 18
n Comet 2P/Encke: UT 2017-02-21
SIRPOL of IRSF (the Infra-Red Survey Facility)n Three channels (JHKs) simultaneous polarimetry with wide FoV (7.7’x7.7’)
n Comet 252P/LINEAR: UT 2016-03-03 – 15n The 5th closest comet to Earth in history, passing directly over the Antarctic
MSI (Multi-Spectral Imager) of Pirka n Comet 252P/LINEAR: UT 2016-04-03 (Rc-band)Results & Discussion
1. Unimodal Distribution of Linear
Polarization degree (P) of Cometary dust

3. Results & Discussion

P of Comet Dust
High Pmax comet(a.k.a. dust-rich)

Low Pmax comet(a.k.a. gas-rich)

Since, Levassuer-Regourd+1996 suggest the dichotomy of polarization degree,

Real difference of bulk

properties of dust particles!

Contamination of

depolarizing gas emissions!

3. Results & Discussion

P of Comet Dust
Multi-band Imaging(g’, RC, IC)
Observations at OAO

Optical spectroscopy(0.58 – 1.00 μm)
Observations at HHO

Optical + NIR polarimetry(RC, IC, KS)
Observations at HHO

Comet C/2013 US10 (Catalina)o Flux ratio of gas-to-dust signal in RC-bando Double check between two obs. Modeso Mostly NH2 line emissionso Derived P of “dust” by applying NH2polarization valueso Pobs = Pdust X fdust + Pgas X fgas
(fgas + fdust = 1)
http://www.cometwatch.co.uk/comets-of-2015/c2013-us10-catalina/

3. Results & Discussion

1) Unimodality of P(α)
R-filter domain

Polarization Degree [%]30

20

Increments of P after gas correction islarge enough to cover the difference
between two classes.

C/1995 O1 (Hale−Bopp)
1P/Halley
109P/Swift−Tuttle
C/1987 P1 (Bradfield)
C/1989 X1 (Austin)
C/1996 B2 (Hyakutake)
C/2013 US10

Spectral dependence of gas-corrected
P shows almost neutral color.

14

10

12
10
8

0
0

48 49 50 51 52 53 54 55

20

40
60
80
Phase Angle [Deg]

100

120

After gas correction, spatial variationof P nearly vanishes within its coma.
High-Pmax class-like behaviors!

n We found that low Pmax (gas-rich) comets are observationally biased
results due to the gas depolarization.
n Despite of various cometary species, their phase angle dependence ishomogeneous. This supports common origin of cometary dust particles.

However,Results & Discussion
2. Dominance of Large-sized Dust
Amongst Cometary Species

3. Results & Discussion

Comet 2P/Encke and 252P/LINEAR, Lovely outliers

2P/Encken Discovered in 1786
n Porb=3.3 yrs with e=0.84

252P/LINEARn The 5th closet comet to Earth inhistory (≈0.03 au)

n Optically gas-rich, but heavily dust- n Ejected very recently (≈300 yrs)
rich in IR
to the Near-Earth Orbital regionn Detached orbit from Jupiter! Most likely located on the final
dormant stage of evolution..

n But looks like almost dead!

3. Results & Discussion

P-Anomaly of 2P/Encke
The most recent polarimetric status
in terms of 2P/Encke’s current evolutionary stageo Spectropolarimetry at α=75.7° on UT 2017
FEB 21o P and P-color of dust and gas components

Albedo

30
20
10

3200
209P

2P

Maximum polarization degree (Pmax) =41.7±1.7 % at αmax ≅ 99.5°
M = 160 μmd

Md = 85 μm
Md = 60 μm
Md = 25 μm high-Pmax
Md = 12.5 μm
Md = 2.5 μm

NH2(0,7,0)

0
0

(b)

0.1

st

40

(a)

This study
Jockers et al.(2005)
Jewitt (2004)
Kiselev et al.(2004)
Pmax = 41.7 %αmax = 99.5°

Du

Polarization Degree [%]50

o P-angle (θP) of both components and theirwavelength dependence

20

40
60
80
Phase Angle [deg]

100

120

10

,which is nearly 14 % higher than any
other comets!
Pmax [%]

3. Results & Discussion

20

0

20

40
60
80
Phase Angle [deg]120

From g’-Rc-Ic multi-band imaging data taken by
Pmax [%]
Ishigakijima Astronomical Observatory
(b)

10

(a)

3200

st

AlbedoThe medium
0.1
size of dust
0.08
grains (Md) of
2P/Encke is
0.06
approximately
an order of
0.04
magnitude
NH (0,7,0)
larger thanother80comets!
60
100
120
10
2

40
Phase Angle [deg]

100

2P

Md = 160 μm
Md = 85 μm
Md = 60 μm
Md = 25 μm high-Pmax
Md = 12.5 μm
Md = 2.5 μm

NH2(0,7,0)

his study (dust, NH2)
ockers et al.(2005) (dust, NH2)
witt (2004) (dust)
elev et al.(2004) (NH2)41.7 %
99.5°

du

209P

10
0

20

3200

Albedo

30

(b)
0.1

st

40

(a)

This study
Jockers et al.(2005)
Jewitt (2004)
Kiselev et al.(2004)
Pmax = 41.7 %αmax = 99.5°

Du

Polarization Degree [%]50

P-Anomaly of 2P/Encke

2P
209P
Md = 160 μm
Md = 85 μm
Md = 60 μm
Md = 25 μm high-Pmax
Md = 2.5 μm

2030
Pmax [%]

40

50

3. Results & Discussion2) Aging of Cometary Dust
Influence of the Dominance of Large-sized dust Particles amongst Cometary Species
Sintering-induced growth

Mangkonsu et al.(2014)

Favorable orbital motion
Highly elongated orbit withvery short orbital period +perihelion distance‘quasi’-invariant parameter relative to Jupiterunder the “restricted circular three-body” problem

4. Summary

In Summary,We have performed polarimetric observations of comets
To study the evolutionary trend of cometary dust particles.
First, we found the unimodal distribution of polarization degree of cometary
dust by showing the observationally biased trend of the gas-rich comet,which supports the common origin of cometary dust.
However, we found the polarimetric anomaly of dynamically very old Comet
2P/Encke via spectro-polarimetry, which is nearly 14 % higher than any other cometary species.
Based on the relationship with observed polarimetric – dynamical propertiesof small bodies,
We conjecture that favorable orbital motion of small bodies for the growthof large-dust particles over the surface can engender the evolutionary
differentiation.

ynkwon@astro.snu.ac.kr

1. Introduction

Polarimetry of Cometary Dust Particles
Iㅗ
Scattering Plane

I//

Scattered sunlight reflects
the physical characteristics
of a single scatter in
diffuse media likecometary coma.
(Rosenbush, et al. 2007)

α: Phase Angle

Spectro-Polarimetry modules
Polarimetry modules

Half-wave plate (HWP)
Alters the P-state of a lightwave travelling through this(to linearly polarized light)

Wollaston prism
Separates incident light into
two orthogonal linearlypolarized outgoing beams

Grism
Makes light at a chosen
central wavelength passstraight throughFluorescence Anisotropyn The E of light interacts with the electrons of a molecule. The electrons
are bound to the molecule and thus restricted in their movement.
Typically, there is a direction within the molecule where the electrons
can move more easily. This axis is called ‘the electric dipole moment μ’of the molecule.
n Light is absorbed by the molecule if E of the incoming light is properlyoriented relative to μ of the molecule.

Maximum absorption occurs,
If E and μ are parallel to each other.

No absorption occurs,
If E and μ are normal to each other.

Fluorescence Anisotropyn The probability of the absorption is proportional to the cosine
squared of the angle θ between the direction of E of the exciting
light (= polarization direction) and μ.
n Thus, if light of a particular E orientation (plane polarized light)
impinges on a sample, only those molecules which are properlyoriented relative to this E can absorb the light.

Incoming verticallypolarized light

No absorption

Maximum
absorption

Some of the moleculeswith this orientation
will absorb lightPhotoselectionn When we excite an ensemble of randomly oriented molecules with planepolarized light, we are performing a photoselection process, creating a populationof excited molecules which nominally have their excited dipoles lines up with thepolarization direction of the excitation.

Schematic of
Photoselection effectPotential dipoles

Polar plots ofDipole polulations

Excited state dipoles

Absorption & Excited State Dipole
n Upon absorption of a photon, the electrons of theexcited molecule redistribute. This leads to an excitedμ, μE, in the molecule.
n The orientation of this μE relative to the nuclear
framework, and its magnitude, will be determined by
the nature of the substituents on the molecule.
n This μE is also known as the transition dipole ortransition moment.

n ϕ is the angle between absorption and emission
dipoles.

Fluorescence Emission from an Excited Moleculen Consider an excited molecule withμE that returns to the ground state
via emission of a photon.
n The direction of emission is not
random, but follows a sine squared
law of electric dipole radiation.
n Emission probability ∝ ϒ2

Dipole radiation pattern

Fluorescence Polarization
1)2) Öhman, Y. 1941
: An evidence that cometary
molecular emission is due tofluorescence, and not, for instance,
to thermal excitation.
3) Phase angle distribution of P
C2, C3, CN: good agreement with that
expected from theory
OH: significantly smaller than that ofother molecular species

4) Important factor of Pgas (u ! l)Larmor frequency of the molecule (νL)
Compared with
Einstein coefficient (Auàl) & excitation rate (glàu)• If 2πνL > Auàl (strong magnetic field):
Isotropic & Unpolarized• If 2πνL > Auàl (weak magnetic field):
Alignment of the molecules5) For typical visible molecular bands ofcomets,Auàl ~ 106 s-1, glàu ~ 10-3 – 10-1 s-1(negligible collision rates)
According to the in-situ information of themagnetic field of 1P/Halley, νL ~ 102 s-1.
è gl!u < 2πνL < Au!l6) Linearly polarized fluorescence reemission: In this case, and for unidirectional excitation
by natural lightFluorescence Polarization2) Öhman, Y. 1941
: An evidence that cometary
molecular emission is due to
Polarization dependence on Phase angle
fluorescence, and not, for instance,
to thermal excitation.

5) For typical visible molecular bands ofcomets,Auàl ~ 106 s-1, glàu ~ 10-3 – 10-1 s-1(negligible collision rates)
Accordingto the in-situ
information
Total intensity
on Phase
angle of themagnetic field of 1P/Halley, νL ~ 102 s-1.
è gl!u < 2πνL < Au!l

Fluorescence Polarization
Left
Feofilov, 1959
(Extension of Öhman, 1941)
RightÖhman, 1941
(Extension of Mrozowski,
1939)

n Three possible limit values are reached for large Js, depending on the kind offluorescence process:q 1/13 (7.7 %) for P and R excitation followed by P or R emission;
q 1/3 (33.3 %) for resonance Q lines;
q -1/7 (14.3 %) for a process involving a Q line and a (P or R) line

Fluorescence Polarizationn Three possible limit values are reached for large Js, depending on the kind offluorescence process:q 1/13 (7.7 %) for P and R excitation followed by P or R emission;
q 1/3 (33.3 %) for resonance Q lines;
q -1/7 (14.3 %) for a process involving a Q line and a (P or R) linen Current polarimetry does not resolve molecular bands.
n For Σ-Σ or Π-Π electronic bands, Q transitions are very weak à thelimiting P_max value is 7.7 %.
q CN violet (0.388 µm) B2Σ+ -- X2Σ+ , C2 (0.514 µm) d3Πg – a3Πun For Σ-Π or Π-Σ bands, the situation is different.
q CN red (0.78 – 1.1 µm) A2Π – X2Σ+, C3, OH, NH2 ammonia band, etc.

No result at all

Few results, saying the possibility of
Systematically (slightly) lower Pmax

2. Observations

HONIR (Hiroshima Optical and Near-InfraRed Camera)
http://hasc.hiroshima-u.ac.jp/instruments/honir/index-j.html

Incident lightHalf-wave plate (HWP)
Alters the P-state of a light wave travellingthrough this (to linearly polarized light)
Wollaston prism
Separates incident light into two orthogonal linearly polarized outgoing beams
Grism
Makes light at a chosen central wavelength pass straight throughUT DateUT 2017 February 21

rh, Δ

0.54 au, 0.97 au

Phase angle (α)

75.7 deg

λ-coverage

0.58 – 1.00 μm

1. Introduction: Method

Spectropolarimetry(2)(1)(Ie, Io)0°, 45°,22.5°, 67.5°

λTo study the properties of cometary constituents free fromcontamination of each other
Polarization degree (P) and P-color of dust and gas components
Polarization angle (θP) of dust and gas components
And their wavelength dependence!
The most recent polarimetric study
in terms of 2P/Encke’s current evolutionary status

3. Results

1) General Appearance
Very weak cont.

log(FNH2/FCN)≅-0.55

NH2-poorP(α) = 33.8±2.7 %P(λ) =-9.9±8.5 % μm-1

Deviation ofΔθP< 4°
Average θP = 0.4°

3. Results2) Dust Polarization

20

209P

10

0

Md = 160 μm
Md = 85 μm
Md = 60 μm
Md = 25 μm high-Pmax
Md = 12.5 μm
Md = 2.5 μm

NH2(0,7,0)

0
20

40
60
80
Phase Angle [deg]3200

Albedo

30

0.1

st

40

(a)

This study
Jockers et al.(2005)
Jewitt (2004)
Kiselev et al.(2004)
Pmax = 41.7 %αmax = 99.5°

Du

Polarization Degree [%]50

Scattered (reflected + refracted+ deflected + reemitted) sunlight by dust
(b)

100

120

10

2P

Penttilä et al. (2005)

Pmax [%]n Using our result (Pdust(75.7°)=33.8±2.7 %) and published data measured withnarrowband filters, we derived 2P’s polarimetric behavior as a function of α ina classical manner.
Maximum polarization degree (Pmax) = 41.7±1.7 % at αmax ≅ 99.5°,which is nearly 14 % higher than any other comets!

3. Results3) Molecular Polarization(a) CN(2,0)
40

R2 (3.5->10.5)

Q2 (2.5->11.5)

Q1 (5.5->15.5)(b) CN(3,1)
40

Q2 (2.5->11.5)

Q1 (5.5->15.5)

3.8±1.2 %CN(1,0)5.8±0.7 %CN(2,1)18.5±3.1 %(b) CN(3,1)
40

10
0.786

(c) CN(1,0)
40

0.788

Q2 (2.5->12.5)0.790
λ [μm]0.792

Q1 (3.5->14.5)

1000.916 0.917 0.918 0.919 0.920 0.921
λ [μm](e) NH2(0,7,0)40

20

PCN [%]

NH2 α30ammonia20band
Q2 (2.5->12.5)00.810 0.811 0.812(0,7,0)0.808 0.809 4.0±1.0%
λ [μm]0.792

Q1 (3.5->14.5)(d) CN(2,1)
40(0,6,0)30(0,10,0)20

00.916 0.917 0.918 0.919 0.920 0.921
λ [μm](e) NH2(0,7,0)
40

30

20

20

0
0
0.6635 0.6640 0.6645 0.6650
0.7010λ [μm](g) NH2(0,10,0)
40

10

20

0.9395 0.9400 0.9405 0.9410 0.9415
λ [μm](f) NH2(0,6,0)400.7015λ [μm]

0.702030300

7.0±0.8 %0.9395 0.9400 0.9405 0.9410 0.9415
λ [μm](f) NH2(0,6,0)
40

0

10

R1 (9.5->7.5)

10Theoreticalprediction= 7.2 %010

R1 (9.5->7.5)

3.7±0.5 %Q2 (2.5->7.5)

10

Q2 (2.5->7.5)

PCN [%]

PCN [%]

200.790
λ [μm]

30

0.808 0.809 0.810 0.811 0.812
λ [μm](d) CN(2,1)
40

30

0.788

10

PNH2 [%]20

0

Q1 (3.5->14.5)

0.786(c) CN(1,0)
4030PCN [%]

PCN [%]30

Q2 (1.5->11.5)

0

PNH2 [%]

R2 (3.5->10.5)

CN(3,1)

Fluorescence anisotropy20(Photoselection
effect) + transitional dipole
moment10
10
20

CN-RedPNH2 [%](a) CN(2,0)
408.4±1.3 %PCN [%]

CN-redsystem(A2Π-X2Σ-)

CN(2,0)Q1 (3.5->14.5)

30

PCN [%]

PCN [%]30

Q2 (1.5->11.5)

20

NH2 ammonia

1000.7365 0.7370 0.7375 0.7380
λ [μm]

4. Discussion

1) P of NH2 & CN-red Molecules

20
10

0

n Q (ΔJ=0), P (+1), R (-1) branches

Md =
Md =
Md =
Md =
Md =
Md =

NH2(0,7,0)0

Le Borgne (1987)

Albedo

30

0.1

st

40

(a)

This study
Jockers et al.(2005)
Jewitt (2004)
Kiselev et al.(2004)
Pmax = 41.7 %αmax = 99.5°

Du

Polarization Degree [%]

50

20

40
60
80
Phase Angle [deg]

100

120

with Pmax = 7.7 %o C2, CN-violet (0.388 μm), C3 = P/R branches dominatedo CN = Q branch dominatedo NH2 = Three vibrational mode (to be considered by symmetric,
bending, asymmetric stretching modes)

104. Discussions

2) Polarimetric Anomaly of 2P Dust

20

209P

10

0

20

40
60
80
Phase Angle [deg]

100

120

2P

Md = 160 μm
Md = 85 μm
Md = 60 μm
Md = 25 μm high-Pmax
Md = 12.5 μm
MFromg’-Rc-Ic multi-band imaging data taken by
d = 2.5 μm

NH2(0,7,0)

0

3200

Albedo

30

(b)
0.1

st

40

(a)

This study
Jockers et al.(2005)
Jewitt (2004)
Kiselev et al.(2004)
Pmax = 41.7 %αmax = 99.5°

Du

Polarization Degree [%]50

10Ishigakijima Astronomical Observatory

Pmax [%]

1. Disintegration of Dustn Jewitt (2004) suggests that the disintegration of dust in
outward movement from the nucleus produces anincrease in P of 2P.
n This hypothesis cannot explain the P-anomaly of 2P.

4. Discussions

2) Polarimetric Anomaly of 2P Dust

30
30
20
20

(b)
(b)
0.1
0.1
209P
209P

10
10

00

20
20

40
40
60
60
80
80
Phase
PhaseAngle
Angle[deg]
[deg]

100
100

120
120

2P
2P

μm
MMd d==160160μmμm
MMd d==8585μm
MMd d==6060μmμm
MMd d==2525μmμm high-P
high-Pmax
max
MMd d==12.5
12.5μmμm
MMd d==2.5
2.5μm
μm

2(0,7,0)NH
NH2(0,7,0)

00

3200
3200

AlbedoAlbedo

40
40

(a)
(a)

This
Thisstudy
study
Jockers
Jockersetetal.(2005)
al.(2005)
JewittJewitt(2004)(2004)
Kiselev
Kiselevetetal.(2004)
al.(2004)
PPmax
41.7%
%
max==41.7
99.5°°
ααmax
max==99.5

Du
D
usst
t

Polarization
Polarization Degree
Degree [%]
[%]

50
50

10
10

PPmax
[%]
max[%]

2. Dominance of Big Particles

n We utilized the lunar regolith sample of different size to observe the behaviorof 2P as well as other small bodies using the Pmax – Albedo relationship.
n Using the relation of Shkuratov’s, we also double checked Md ≈ 61.6 μm.

4. Discussions

2) Polarimetric Anomaly of 2P Dust

20

209P

10

0

Md = 160 μm
Md = 85 μm
Md = 60 μm
Md = 25 μm high-Pmax
Md = 12.5 μm
Md = 2.5 μm

NH2(0,7,0)

0
20

40
60
80
Phase Angle [deg]3200

Albedo

30

(b)
0.1

st

40

(a)

This study
Jockers et al.(2005)
Jewitt (2004)
Kiselev et al.(2004)
Pmax = 41.7 %αmax = 99.5°

Du

Polarization Degree [%]

50

100

120

10

Pmax [%]

2P

Mangkonsu et al.(2014)

2. Dominance of Big Particles

n Significantly high dynamical tensile strength of particles coming from theTaurids and Geminids (compared to the typical value of ~104 dyn cm-2)
n The sintering of ice grains leads to a continuous growth of the grain-to-grain
contact areas and condensation of them (Kelvin effect) (Kossacki et al. 1994).

1. Progress Report

Backward Simulation