Journal of The Korean Astronomical Society (천문학회지)

The Korean Astronomical Society (한국천문학회)
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THE GEOMETRIC ALBEDO OF (4179) TOUTATIS ESTIMATED FROM KMTNET DEEP-SOUTH OBSERVATIONS

  • Bach, Yoonsoo P. (Department of Physics and Astronomy, Seoul National University) ;
  • Ishiguro, Masateru (Department of Physics and Astronomy, Seoul National University) ;
  • Jin, Sunho (Department of Physics and Astronomy, Seoul National University) ;
  • Yang, Hongu (Korea Astronomy and Space Science Institute) ;
  • Moon, Hong-Kyu (Korea Astronomy and Space Science Institute) ;
  • Choi, Young-Jun (Korea Astronomy and Space Science Institute) ;
  • JeongAhn, Youngmin (Korea Astronomy and Space Science Institute) ;
  • Kim, Myung-Jin (Korea Astronomy and Space Science Institute) ;
  • Kwak, SungWon (Korea Astronomy and Space Science Institute)
  • Received : 2019.01.25
  • Accepted : 2019.05.21
  • Published : 2019.06.30
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Abstract

We derive the geometric albedo of a near-Earth asteroid, (4179) Toutatis, to investigate its surface physical conditions. The asteroid has been studied rigorously not only via ground-based photometric, spectrometric, polarimetric, and radar observations but also via in situ observation by the Chinese Chang'e-2 space probe; however, its geometric albedo is not well understood. We conducted V-band photometric observations when the asteroid was at opposition in April 2018 using the three telescopes in the southern hemisphere that compose the Korea Microlensing Telescope Network (KMTNet). The observed time-variable cross section was corrected using the radar shape model. We find that Toutatis has a geometric albedo $p_V=0.185^{+0.045}_{-0.039}$, which is typical of S-type asteroids. We compare the geometric albedo with archival polarimetric data and further find that the polarimetric slope-albedo law provides a reliable estimate for the albedo of this S-type asteroid. The thermal infrared observation also produced similar results if the size of the asteroid is updated to match the results from Chang'e-2. We conjecture that the surface of Toutatis is covered with grains smaller than that of the near-Sun asteroids including (1566) Icarus and (3200) Phaethon.

Keywords

CMHHBA_2019_v52n3_71_f0001.png 이미지

Figure 1. Summary of the data reduction process performed by teams at the Korea Astronomy and Space Science Institute (KASI) and Seoul National University (SNU). Photometry was conducted with publicly available open-source packages indicated at the bottom left (the shadowed box), which includes astroquery (Ginsburg et al. 2018), astroscrappy, ccdproc (Craig et al. 2017), reproject (Robitaille 2018), and photutils (Bradley et al. 2017), in addition to the astropy core package (Astropy Collaboration et al. 2013, 2018).

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Figure 2. Reduced magnitude, HV(α), and phase-corrected magnitude, HV, as function of time. The numbers above each marker indicate the phase angles α at the observation epoch. The red solid line is the best-fit geometrical crosssectional area, −2:5 log10(Sproj(t)) (See Equation 5), the faint black dotted lines indicate model curves within the 1-σ confidence interval (See Appendix A). The short-time (~1−2 days) bumpy features in the data are reproduced in the model, meaning that these features are attributed to the rotation of the asteroid.

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Figure 3. Top: The radiance factor (I/F) as a function of phase angle (α). The observed radiance factor values (blue cross markers) are tted using a single power-law: I/F(α) = pV × 10bα. The best fit pV, f1, and f2 (see Equation 9) and their 1-σ uncertainties are shown above the figure. The red solid line is the best-fit model, the shaded area marks the 1- con dence interval of the model fit. The black marker shows A5 = I/F(α = 5°) and its 1-σ confidence interval, indicated by the numbers next to the marker. Bottom: The corresponding $X^{2}_{red}$ statistic in the parameter space. The 1-σ confidence contour of $X^{2}_{red}$ and that of pV are shown. The colormap is in log-scale and contours for a few selected of$X^{2}_{red}$ values are shown to guide the eyes.

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Figure 4. Albedo at α = 5° vs. Pmax plot for lunar and terrestrial samples in Geake & Dollfus (1986) along with those of three asteroids whose Pmax values are well studied. The data for (3200) Phaethon and (1566) Icarus are from Ishiguro et al. (2017) and Ito et al. (2018), respectively. The word "Terr" in the legend means "terrestrial" materials. The numbers near the three asteroidal objects denote the perihelion distances of each asteroids in au.

Table 1 Symbols frequently used in this paper

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Table 2 Summary of our observations

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Table 3 Parameter values derived using either all observational data or after excluding the data of 2018-04-07 CTIO (α = 2:79°)

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Table 4 Geometric albedo (pV) values obtained from polarimetry

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