Near Infra-Red Polarimetry of Large Asteroids using NIC
Yoonsoo P. BACH1, 2, Masateru. ISHIGURO1, 2, and Jun Takahashi3
1Nayuta Users' Meeting 2021

Dept of Physics and Astronomy, Seoul National University, 2 SNU Astronomy Research Center, Seoul National University,
3
Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo

Scientific Backgrounds

Results & Discussions

1. PPC (Polarization Phase Curve) :"Polarization (𝑃! ≔ − ) as a#function of phase angle.

The PPC of (4) Vesta à• Linear-exponentialfunction fitting (J, H, Ks):
𝑃! = 𝐴(𝑒 ./0−1) + 𝐵𝛼

2. Umow law [1]:(1) 𝐴 ↑ ⟺ |𝑃min| ↓ (shallower)(2) 𝐴 ↑ ⟺ ℎ ↓ (for 𝐴 ≳ 0.05)(3) 𝐴 ↑ ⟺ 𝑃max ↓𝑃( = − 𝑞( [%]↓ Confirmation of Umow
(1) 𝐴 ↑ ⟺ |𝑃min| ↓ (shallower)(2) 𝐴 ↑ ⟺ ℎ ↓ (for 𝐴 ≳ 0.05)

𝐴 = albedo

3. Observations: particle size on asteroid (regolith)
Large asteroids (including Vesta) are expected to be covered with fineparticles (𝑫 ≲ 𝟐𝟎 µm size) due to large gravity [2-5].

𝜶𝑷𝐦𝐢𝐧

4. Experiments: Particle size from experiments
𝑃min - 𝛼min and 𝑃min - 𝛼$ both showed abrupt change when 𝑫/𝝀 gets ≲ 𝟓 [6]𝒉✅ Confirmation ofUmow law (1)𝐴 ↑ ⟺ |𝑃min| ↓(shallower)

𝐷/𝜆(𝐴)

𝐷/𝜆(𝐴)

𝑫~𝟓𝝀𝑃 min , 𝛼 min , 𝛼 $calculated by
cubic spline with
s=0.018 to theoriginal data.

𝑫~𝟓𝝀✅ Confirmation ofUmow law (2)𝐴↑⟺ℎ↓(for 𝐴 ≳ 0.05)

Geom. Alb. [%]Geom. Alb. [%]5. Motivation & Strategy
Observe the largest available asteroids, (4) Vesta and (1) Ceres, with as longwavelengths as possible. But if 𝜆 ≳ 5 µm, thermal radiation from the asteroid
comes in, so we have to choose 𝜆 ≲ 3 µm. Check from which λ the expected"abrupt" change in PPC happens. So far, there is no refreed publication which
studied NIR polarimetry of asteroids.

𝑷𝐦𝐢𝐧

𝑷𝐦𝐢𝐧
𝑫≫ 𝟏𝟎
𝝀IC (λ 0.8 µm)

J (λ 1.2 µm)

H (λ 1.6 µm)

Ks (λ 2.2 µm)

20 µm

3625

1713910 µm
5 µm

189136846352to small 𝑫/𝝀𝑫≫ 𝟏𝟎𝝀𝑫~𝟓𝝀observation in this work

size in wavelength values (𝐷/𝜆)Particle size 𝐷 V (λ 0.55µm)

Abrupt change when 𝑫/𝝀 gets ≲ 𝟓?
Using archived optical polarimetry data[10], R band (0.60-0.70 µm) is the mostreliable among them. Plotting R & JHK bands simultaneously, we find "abrubtchange" in the 𝑃min - 𝛼min and 𝑃min - 𝛼$𝑫~𝟎. 𝟏
𝝀𝑫~𝟎. 𝟏
𝝀
𝑫~𝟏
𝝀

𝑫~𝟓?
𝝀𝑫~𝟓?
𝝀𝜶𝟎𝜶𝐦𝐢𝐧Vesta (Dawn)
Floronia quadrangle

𝑫~𝟏
𝝀Comparing our R-J-H-Ks data points with the previous experimental works, wesuggest the abrupt change may indicate the criterion 𝑫/𝝀~𝟓 happens near theH and Ks bands. Therefore, λ ~ 2 µm, and we find the particle size on asteroid(4) Vesta, 𝑫~𝟏𝟎µ𝐦. This is not a lower/upper bound, but an exact value.

https://solarsystem.nasa.gov/resources/1755/hamo-and-lamo-images-of-licinia-crater/

2 km

10 km

If the experimental work is applicable to the realistic asteroidal materials, this isa novel and very reliable measurement of particle sizes on an asteroid.
Further experimental studies and NIR polarimetry on large asteroids may berequired to understand the particle size determination on asteroidal regoliths,
such as the determination of the critical 𝐷/𝜆 value (we used ~ 5) depending onmaterials and porosity. If our results of "abrupt change" is a true feature, theories
to describe it must also be developed.

Observation

Conclusion

We found NIC[7-9] is a perfect instrument for our purpose, since it takes three (J,H, Ks) polarimetric images simultaneously, and thus sampling three sizes (𝑫/𝝀)at the same time!

We conducted the near infrared polarimetric observation of a large asteroid (4)Vesta. As expected from the experimental studies, we confirmed Umow laws. So
far, near infrared polarimetry of asteroids has never been published to any peerreviewed journal to our best knowledge. Furthermore, we may have detected the"abrupt change" in the polarimetric phase curve of asteroid due to the size
parameter (size/wavelength), which has only been confirmed in laboratory
experiments. If we assume the critical size parameter where such abrupt change
occurs in experiments can be applied to asteroids, we can conclude the particle
size on asteroid (4) Vesta is about 10 µm.

Mid UT(ISO 8601)

phase angle
𝜶 [°]EXPTIME[s]

AIRMASS𝒓𝐡[au]

𝒓𝐨[au]

lunar
elong. [˚]Vmag

2019-10-22
15:41:39.1
2019-11-08
15:37:37.7

10.4°

2

1.09-1.13

2.54

1.61

70

6.8

4.1°

3, 5,
10, 301.42-1.67

2.55

1.57

60

6.5

2019-11-21
14:13:34.0

5.7°

2

1.12-1.24

2.55

1.58

120

6.6

2019-12-18
14:04:52.4

15.6°

2, 3

1.17-1.47

2.56

1.76

120

7.1

2020-01-10
13:38:53.7

20.7°

21.42-1.67

2.57

2.02

60

7.6

2020-02-13
09:39:28.4

22.5°

21.09-1.13

2.57

2.47

150

8.1Columns: MidUT = Median of the observation for each night; phase angle = the Sun-target-observer's angle; EXPTIME = The exposure times
used for each night's observation; AIRMASS = The minimum and maximum airmass values for each night; 𝒓 𝐡 = heliocentric distance; 𝒓 𝐨 =geocentric (observer-target) distance; lunar elong. = Lunar elongation angle; V-mag = V-band magnitude.
All values, except for EXPTIME, are queried from NASA/JPL/HORIZONS web service (https://ssd.jpl.nasa.gov/horizons.cgi#top).

References[1] Geake, J. E. & Dollfus, A. (1986) MNRAS, 218, 75.; [2] Le Berte T. & Zellner B. (1980), Icar., 43, 172.; [3] Hiroi, T., Pieters,C. M., Takeda, H. (1994), Metic., 29, 394.; [4] Li, J.-Y. et al. (2011), Icar, 216, 640.; [5] Martikainen, J. et al. (2019), MNRAS,483, 1952.; [6] Geake J. E. & Geake M. (1990), MNRAS, 245, 46.; [7] Ishiguro, M. et al. (2011), Annu. Rep. Nishi-Harima
Astron. Obs. 21, 13.; [8] Takahashi, J. et al. (2018), Stars and Galaxies 1, 17.; [9] Takahashi, J. (2019), Stars and Galaxies 2(3), 1. [10] Lupishko, D., Ed. (2019). Asteroid Polarimetric Database V1.0. urn:nasa:pds:asteroid_polarimetric_database::1.0.
NASA Planetary Data System; https://doi.org/10.26033/z7xq-1x46