NIR Polarimetryof theDawn Mission Targets
Hints the Existence ofFine Particles
Yoonsoo P. Bach1,2,3, Masateru Ishiguro2,3, Jun Takahashi4,Jooyeon Geem2,3, Daisuke Kuroda5, Hiroyuki Naito6, Jungmi Kwon7
1. KASI, Korea; 2. Seoul Nat U, Korea; 3. Seoul Nat U, Astronomy Research Center, Korea; 4. Center for Astronomy, U Hyogo, Japan;
5. Japan Spaceguard Association, Japan; 6. Nayoro Observatory, Japan; 7. The U of Tokyo, Japan

Nayuta Users' Meeting 2023

Contents
Ø Intro: Polarimetry in Solar system sciencesØ Motivation & Target selection

Ø Observations & Results◦ NHAO NIC (Nishiharima Infrared Camera)Ø Discussions

Previously:• 2021 – IAU Astropol: Veata• 2021 – Nayuta UM: Vesta• 2022 – Nayuta UM: Vesta & Ceres
Now:2023 – Under review, A&A (2 papers)

Intro: Polarimetry in Solar System
Ø Polarimetric phase curve (PPC)◦ Phase angle α = 180° − scat. ang.
◦ Pol. as phase angle 𝑃! = 𝑃! (𝛼)[%]𝑃! =Fresnel, Rayleigh: 𝑰∥ < 𝑰⟂𝐼⟂𝐼∥Sun𝛼Scattering plane𝐼⟂ − 𝐼∥
𝑄=−𝐼⟂ + 𝐼∥𝐼Obs.

Intro: Polarimetry in Solar System
Ø Polarimetric phase curve (PPC)◦ Phase angle α = 180° − scat. ang.
◦ Pol. as phase angle 𝑃! = 𝑃! (𝛼)[%]𝑃! =Fresnel, Rayleigh: 𝑰∥ < 𝑰⟂𝐼⟂𝐼∥Sun𝛼Scattering plane𝐼⟂ − 𝐼∥
𝑄=−𝐼⟂ + 𝐼∥
𝐼Obs.

Trans. Roy. Soc. Edinburgh
1861, vol 23

Intro: Polarimetry in Solar System
Ø Polarimetric phase curve (PPC)◦ Phase angle α = 180° − scat. ang.
◦ Pol. as phase angle 𝑃! = 𝑃! (𝛼)[%]𝑃! =𝐼⟂ − 𝐼∥
𝑄=−𝐼⟂ + 𝐼∥
𝐼𝛼%&'Neg. Pol. Branch
NPB (𝜶 < 𝜶𝟎 )Fresnel, Rayleigh: 𝑰∥ < 𝑰⟂𝐼⟂
𝐼∥

Sun
𝛼Scattering plane

Pos. pol. Branch
PPB (𝜶 > 𝜶𝟎)
non-flatness
information
width 𝛼#depth 𝑃!"#Obs.

Motivation[1] Robinson et al. (2001) Nature, 413, 396

Ø Particle size 𝐷/𝜆 ≫ 10 (geometric regime).
◦ 例) Eros: Smallest D ≲ 50 µm at "ponds" [1]◦ Say D=10 µm fixed◦ Increase λ = 0.5 µm (V) → 2 µm (Ks)?
𝐷/𝜆 =20

5𝑃! =width 𝜶𝟎

Depth𝑷𝐦𝐢𝐧

𝐷=𝜆V HRI JKλ𝐼⟂ − 𝐼∥𝑄=−𝐼⟂ + 𝐼∥
𝐼𝛼%&'Neg. Pol. Branch
NPB (𝜶 < 𝜶𝟎 )Pos. pol. Branch
PPB (𝜶 > 𝜶𝟎)
non-flatness
information
width 𝛼#depth 𝑃!"#Geake & Geake (1990) MNRAS

Targets
Criteria:• Brightest• Most studied• Likely to contain small (≲ 20µm) particles(4) Vesta
560 × 450 km"Airless Bodies"(1) Ceres
960 × 890 km

Observation

Special Thanks

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

3+ year of data reduction→ Published NICpolpy

Bach et al. 2022 (SAG)

Mid UT(ISO 8601)

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

𝒓𝐨[au]

lunar
elong. [˚]Vmag

2019-10-22 15:41:39.1

10.4°

2

1.09-1.13

2.54

1.61

70

6.8

2019-11-08 15:37:37.7

4.1°

3, 5,
10, 301.42-1.67

2.55

1.57

60

6.5

Results
Ø Ceres data matches very well with previous work (Masiero et al. 2022 PSJ)Ø The first NIR polarimetry of V-type (Vesta)

The first NIR pol. for asteroids (!?)

Bach et al. 2023b (A&A under review)

Results
Ø Ceres data matches very well with previous work (Masiero et al. 2022 PSJ)Ø The first NIR polarimetry of V-type (Vesta)

The first NIR pol. for asteroids (!?)

Bach et al. 2023b (A&A under review)

Results

Large rocks

Depth𝑷𝐦𝐢𝐧

FineParticles(compared to λ)width 𝜶𝟎

Depth𝑷𝐦𝐢𝐧

Data: Geake et al. (1986) MNRAS

λwidth 𝜶𝟎
Bach et al. 2023b (A&A under review)

Geake & Geake (1990) MNRAS

ResultsLarge rocksØ 𝑷𝐦𝐢𝐧 - 𝜶𝟎 change for Vesta
Ø Particle size:Vesta

𝐷~ 5 − 10 𝜆 ~ 2µm
𝜆𝑫 ~ 𝟏𝟎 − 𝟐𝟎 µ𝐦

Bach et al. 2023a,b (A&A under review)𝐷≲ 5𝜆Depth𝑷𝐦𝐢𝐧

FineParticles(compared to λ)width 𝜶𝟎Depth𝑷𝐦𝐢𝐧

𝐷
≲ 5𝜆

λ

Ceres
Data: Geake et al. (1986) MNRAS

width 𝜶𝟎
Bach et al. 2023b (A&A under review)

Discussions
Ø Change in Vesta
Ø Mechanisms other than D/λ?
◦ Albedo: not likely◦ Roughness/Porosity: N/A◦ Refractive index: possible,but less preferred§ Occam's razor

Ø D/λ effect is the only mechanismthat explains both
◦ experiments (Geake & Geake 1990) and◦ our observation.

Bach et al. 2023a (A&A under review)

Discussions – ImportanceØ First application of NIR polarimetry to quantify grain size.
Ø Compensates thermal modeling based estimations.

Thermal modeling

Data from MacLennan & Emery (2022) PSJ
Figure from Bach, Y. P. (2023) PhDTFuture Works – Vesta[1] Reddy V. et al. (2012) Science𝑷𝐫Ø Disc-resolved NIR polarimetry

largerparticle

Ø Vesta is one of the "most heterogeneous asteroids"[1]smallerparticle

𝜶Subaru IRCS with AO188 (K-band)(Best expected resol. = 70 km)~ 560 kmglobalaverage(Bach+23)

Vesta K-band 𝑷𝒓~ 400 kmcolor ~ composition

Small particle
high albedo region

Large particle
high albedo region

Small particle
low albedo region

Large particle
low albedo region

https://trek.nasa.gov/vesta/#

2023B proposal rejected
Planning to submit again

Crater ejecta (high albedo, 50%)

Dark region (low alb