Nayuta Users' Meeting 2022

Near-IR Polarimetryof Large Atmosphereless Bodies(1) Ceres & (4) Vesta
YOONSOO P. BACH1, MASATERU ISHIGURO1, JUN TAKAHASHI2, JOOYEON GEEM1,HIROYUKI NAITO3, JUNGMI KWON4, AND DAISUKE KURODA5
1 D EPARTMENT OF P HYSICS AND A STRONOMY , S EOUL N ATIONAL U NIVERSITY , S. K OREA ; SNU A STRONOMY R ESEARCH C ENTER (SNUARC), D EPARTMENT OF P HYSICSAND A STRONOMY , S EOUL N ATIONAL U NIVERSITY , S. K OREA ; 2N ISHI-H ARIMA A STRONOMICAL O BSERVATORY , C ENTER FOR A STRONOMY , U NIVERSITY OF H YOGO , JAPAN ;3N AYORO O BSERVATORY , J APAN ; 4T HE U NIVERSITY OF T OKYO , J APAN ; 5O KAYAMA O BSERVATORY , K YOTO U NIVERSITY , J APAN

Contents
Ø Introduction
Ø Observation
Ø Results
Ø Discussions

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INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Introduction - Asteroid polarimetry
Ø The “proper” polarization degree, 𝑃r ,𝐼" − 𝐼∥
𝑃! ≔𝐼" + 𝐼∥

∥ & ⟂: parallel/perpendicularto the scattering planecf. Rayleigh scattering: 𝐼∥ < 𝐼⟂

is known to be a function of phase angle (𝜶) only (observation & Lab)ie𝛼ψ2022-07-25 Nayuta Users' Meeting

INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Introduction
IshiguroM+ (2022),
MNRAS, 509, 4128

Ø These simple curve gives information on◦ albedo◦ spectral type◦ particle size
◦ hydrated minerals

GeemJ+ (2022),
MNRAS in press

𝑃&'(halbedo
GeakeJE+DollfusA(1986), MNRAS, 218, 75

BelskayaIN+ (2017)
Icar, 284, 30𝑷𝐦𝐢𝐧

h2022-07-25 Nayuta Users' Meeting

𝜶𝟎INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Introduction

observation in this work

Ø Among these, a long-forgotten (?) relation:◦ polarimetry as function of size-parameter (𝝅𝑫𝝀)Particlesize

V(0.55µm)

IC(0.8 µm)

J(1.2 µm)

H(1.6 µm)

Ks(2.2 µm)

20 µm

3625

1713910 µm

18138655 µm

96432to small D/𝝀numbers = D/λ𝜶𝐦𝐢𝐧𝑷𝐦𝐢𝐧𝜶𝟎D/λ=
1–5D/λ=1–5𝑷𝐦𝐢𝐧

Large asteroids are expected to be covered withfine particles (≲ 𝟐0 µm size) due to large gravity
polarimetry [Le Berte T. & Zellner B. (1980), Icar., 43, 172]and spectroscopy [Hiroi, T., Pieters, C. M., Takeda, H. (1994), Metic., 29, 394; Li, J.-Y. et al. (2011),Icar, 216, 640.; Martikainen, J. et al. (2019), MNRAS, 483, 1952.]

λλGeakeJE+GeakeM (1990), MNRAS, 245, 46
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size in wavelength values (𝑑/𝜆)INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Introduction
Ø Particle size measurement is very important yet difficult...
Vesta (Dawn)
Floronia quadrangle
https://solarsystem.nasa.gov/resources/1755/hamo-and-lamo-images-of-licinia-crater/

GrottM+ (2019) NatAs 3 971.

10 km

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2 km

INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Observation
Ø NHAO NIC, polarimetric-mode
Ø J/H/Ks simultaneous polarimetry
Ø (1) Ceres: 5 obs.
◦ 2020-06-21, 2020-10-02, 2020-10-18, 2021-09-09, 2021-11-07

Ø (4) Vesta: 6 obs.
◦ 2019-10-22, 2019-11-08, 2019-11-21, 2019-12-18, 2020-01-10, 2020-02-13

World-FirstSecond* Near-IR polarimetryof asteroid using NHAO NIC.
Found regolith particle size using NIR polarimetry:

IshiguroM+2011, ARNHAO, 21, 13
TakahashiJ+2018, SAG, 1,17
TakahashiJ+2019, SAG, 2(3)* among peer-reviewed publications
MasieroJ+ (2022), PSJ, 3, 90

2022-07-25 Nayuta Users' Meeting

INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSIONMid UT(ISO 8601)

phase angle
𝜶 [°]EXPTIME[s]AIRMASS𝒓𝐡[au]

𝒓𝐨[au]

lunarelong. [˚]Vmag

2019-10-22 15:41:39.1

10.4°

21.09-1.13

2.541.61

70

6.8

2019-11-08 15:37:37.7

4.1°

3, 5,10, 30

1.42-1.67

2.55

1.57

60

6.5

A Justification of Our Data Reduction
Strongly polarized standard star

literaturevalues

H-data are all excluded as we put more
stringent condition for "saturation"(if 5+ pixels are saturated, whole set is discarded)

Takahashi
2019 result2022-07-25 Nayuta Users' Meeting

INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Results
Ø "aperture mode" means different photometric algorithms.
◦ Note how consistent they are, regardless of the algorithms.

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INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Results
Ø Ceres: Two data (both at J/H) from another study coincides well◦ max difference ~ 0.1%p

MasieroJ+ (2022), PSJ, 3, 90

Ø Vesta: THE first result2022-07-25 Nayuta Users' Meeting

INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Results - Ceres
Ø Phase curve at NIR ≈ optical dataØ parameters◦ h𝒉 : slightly increases at longest λ? (0.263 → 0.258 → 0.312 [%/°])◦ 𝛼𝜶$𝟎 : virtually no change (17.9 → 17.7 → 17.5 °)◦ 𝑃: virtually no change (-1.7 → -1.6 → -1.6 %)𝑷%&'𝐦𝐢𝐧◦ 𝛼𝜶𝐦𝐢𝐧%&' : virtually no change? (7.2 → 7.3 → 8.1 °)2022-07-25 Nayuta Users' Meeting

INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Results - Ceres
Ø Phase curve at NIR ≈ optical dataØ parameters◦ h𝒉 : slightly increases at longest λ? (0.263 → 0.258 → 0.312 [%/°])◦ 𝛼𝜶$𝟎 : virtually no change (17.9 → 17.7 → 17.5 °)◦ 𝑃: virtually no change (-1.7 → -1.6 → -1.6 %)𝑷%&'𝐦𝐢𝐧◦ 𝛼𝜶𝐦𝐢𝐧%&' : virtually no change? (7.2 → 7.3 → 8.1 °)Ø Consistent with previous work
MasieroJ+ (2022), PSJ, 3, 90.

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INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Results - Vesta
Ø Phase curve at NIR ≠ optical dataØ parameters◦ h𝒉 : increases as wavelength (0.060 → 0.071 → 0.096 [%/°])◦ 𝜶𝜶𝟎𝟎 : increases as wavelength(22.9 → 23.8 → 24.1 °)◦ 𝑷𝐦𝐢𝐧 : deepens as wavelength(-0.7 → -0.8 → -1.1 %)𝑷𝐦𝐢𝐧◦ 𝜶𝜶𝐦𝐢𝐧𝐦𝐢𝐧 : increases as wavelength (7.2 → 7.7 → 8.3 °)Ø First data

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INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Discussion
Ø Expected results are seen for slope halbedo

albedoGeakeJE+DollfusA(1986), MNRAS, 218, 75

high-albedo
Vesta

low-albedo
Ceresh2022-07-25 Nayuta Users' Meeting

INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSIONhDiscussion
slope-albedo law
holds in Near-IR too!

Ø Expected results are seen for slope halbedo

albedoGeakeJE+DollfusA(1986), MNRAS, 218, 75

high-albedo
Vesta

low-albedo
Ceresh2022-07-25 Nayuta Users' Meeting

INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSIONhDiscussion
Ø New findings

numbers = D/λnumbers = D/λVesta◦ Location for two are
different may be because ofcompositional difference(V-type VS C-type).

Vesta

Ceres

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INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSIONCeres

Discussion
Ø New findings

numbers = D/λnumbers = D/λVesta◦ Location for two are
different may be because ofcompositional difference(V-type VS C-type).

Vesta◦ Ceres:§ No change.
§ Likely D/λ ≫ 5 for all λ§ (𝐷 ≫ 5 𝜆# ~ 10 µm)§ or already 𝐷 ≪ 5𝜆$ ~ 1.5µm.

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Ceres

INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Ceres

Discussion
Ø New findings

numbers = D/λnumbers = D/λVesta◦ Location for two are
different may be because ofcompositional difference(V-type VS C-type).

Vesta◦ Ceres:§ No change.
§ Likely D/λ ≫ 5 for all λ§ (𝐷 ≫ 5 𝜆# ~ 10 µm)§ or already 𝐷 ≪ 5𝜆$ ~ 1.5µm.

Ceres◦ Vesta:§ A sudden change when 𝝀 = 𝝀𝑲
§ D ~ 10 µm??

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INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

Ceres

Discussion

numbers = D/λØ New findings

numbers = D/λVesta◦ Location for two are
different may be because ofcompositional difference(V-type VS C-type).

Vesta◦ Ceres:§ No change.
§ Likely D/λ ≫ 5 for all λ§ (𝐷 ≫ 5 𝜆# ~ 10 µm)§ or already 𝐷 ≪ 5𝜆$ ~ 1.5µm.

Ceres◦ Vesta:§ A sudden change when 𝝀 = 𝝀𝑲
§ D ~ 10 µm??
Large asteroids are expected to be covered withfine particles (≲ 𝟐0 µm size) due to large gravity
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INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSIONCeres

Discussion
Ø New findings◦ Location for two aredifferent may be because ofcompositional difference(V-type VS C-type).

goes deep into
"lunar fine"◦ Ceres:§ No change.
§ Likely D/λ ≫ 5 for all λ§ (𝐷 ≫ 5 𝜆# ~ 10 µm)§ or already 𝐷 ≪ 5𝜆$ ~ 1.5µm.

◦ Vesta:§ A sudden change when 𝝀 = 𝝀𝑲
§ D ~ 10 µm??

Lunar data from GeakeJE + DollfusA (1986) MNRAS 218 75
Experiment data (circles) from GeakeJE+GeakeM (1990), MNRAS, 245, 46
Asteroid data from BelskayaIN+ (2017) Icar, 284, 30

Large asteroids are expected to be covered withfine particles (≲ 𝟐0 µm size) due to large gravity
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INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION

ConclusionØ Nearly world-first NIR polarimetry of asteroidsØ The first observation to see the polarimetric behavior at small 𝐷/𝜆 in Vesta.
◦ Vesta: D ~ 10 µm?
◦ Ceres: inconclusive◦ Strong point: This is not a lower/upper bound, but quite an exact value.

Ø Further investigation needed:◦ Is the lab experiment applicable to real asteroids?

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INTRODUCTION ⟹ OBSERVATION ⟹ RESULTS ⟹ DISCUSSION