Inversion Angle of Phase-polarization Curve of Near-Earth Asteroid (3200) Phaethon

Abstract

The linear polarization degree (referred to the scattering plane, P r ) as a function of the solar phase angle, α, of solar system objects is a good diagnostic to understand the scattering properties of their surface materials. We report P r of Phaethon over a wide range of α from 19 • .1 to 114 • .3 in order to better understanding properties of its surface materials. The derived phase-polarization curve shows that the maximum of P r , P max , is >42.4% at α >114 • .3, a value significantly larger than those of the moderate albedo asteroids (P max ∼9%). The phase-polarization curve classifies Phaethon as B-type in the polarimetric taxonomy, being compatible with the spectral property. We compute the geometric albedo, p v , of 0.14 ± 0.04 independently by using an empirical slope-albedo relation, and the derived p v is consistent with previous results determined from mid-infrared spectra and thermophysical modeling. We could not find a fit to the period in our polarimetric data in the range from 0 up to 7.208 hr (e.g., less than twice the rotational period) and found significant differences between our P r during the 2017 approach to the Earth and that of the 2016. These results imply that Phaethon has a region with different properties for light scattering near its orbital pole.Note-UT date is the mid-time of the start and final sequences. φ and α are the angle of the scattering plane and the solar phase angle of the observations, respectively. Texp is exposure time of each frame in seconds and number of sequences. UP, GT, and SP in the column of polarimetric standard stars indicate unpolarized standard stars, fully linear polarized light from the Glan-Taylor prism, and strong polarized standard stars, respectively; chosen from Serkowski (1974), Schmidt et al. (1992), andWolff et al. (1996) for calibrating the instrumental polarization. RESULTS AND DISCUSSION

Polarimetric properties of the near-Sun asteroid (155140) 2005 UD in comparison with other asteroids and meteoritic samples

“…2011) measured ground and sieved samples whose maximum particle sizes were controlled by their device, while minimum sizes were not, and found that P max depends on the maximum particle size but does not exceed 29 per cent (for Allende, CV3 chondrite with the particle size < 500 μm ). Shinnaka et al 2018 ). This fact suggests that the albedos of these asteroids and carbonaceous chondrites are similar to one another.…”

“…5 ), where h denotes the polarimetric slope parameter (see equation ( 2). In the plot, we included two B-type asteroids, Phaethon (Shinnaka et al 2018 ) and Bennu (Cellino et al 2018 ), for comparison, although Phaethon's P min value has not been determined to date. As we mentioned in Section 3.1, h is a good proxy for geometric albedo, showing that samples with high albedos are distributed leftward, while those with lower albedos are distributed rightward.…”

“…• ). Later, Shinnaka et al ( 2018 ) derived the geometric albedo of p V = 0.14 ± 0.04 via the polarimetric slope and geometric albedo law and found that the geometric albedo is significantly larger than the comet nuclei (Buratti et al 2004 ;Li et al 2009Fern ández et al 2013 ;Kim, Ishiguro & Usui 2014 ;Ciarniello et al 2015 ). Devog èle et al ( 2018) conducted independent polarimetric observations in 2017 and noticed that Phaethon's polarimetric inversion angle, α 0 (the phase angle when the polarization degree is zero) was within the range of typical asteroids but beyond the range of F-type asteroids and cometary nuclei, therefore supporting the idea of asteroidal origin.…”

“…Borisov et al ( 2018 ) utilised a set of data in Devog èle et al ( 2018) and further found that the rotational variation in the polarization degree was probably caused by local heterogeneity. Shinnaka et al ( 2018 ) and Okazaki et al ( 2020 ) pointed out that the polarization degree of Phaethon in 2017 was different from that in 2016 at larger phase angles ( α > 60…”

High-resolution Thermal Infrared Imaging of 3200 Phaethon

“…Even at r H = 1 AU, particles smaller than 5 µm can be expelled. Selective loss of small particles from Phaethon is suggested by polarization studies and could result from this cause (Ito et al 2018), although the interpretation is not unique (Shinnaka et al 2018).…”