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

Abstract

The investigation of asteroids near the Sun is important for understanding the final evolutionary stage of primitive Solar system objects. A near-Sun asteroid, (155140) 2005 UD, has orbital elements similar to those of (3200) Phaethon (the target asteroid for the JAXA’s DESTINY+ mission). We conducted photometric and polarimetric observations of 2005 UD and found that this asteroid exhibits a polarization phase curve similar to that of Phaethon over a wide range of observed solar phase angles (α = 20-105○) but different from those of (101955) Bennu and (162173) Ryugu (asteroids composed of hydrated carbonaceous materials). At a low phase angle (α ≲ 30○), the polarimetric properties of these near-Sun asteroids (2005 UD and Phaethon) are consistent with anhydrous carbonaceous chondrites, while the properties of Bennu are consistent with hydrous carbonaceous chondrites. We derived the geometric albedo, pV ∼ 0.1 (in the range of 0.088-0.109); mean V-band absolute magnitude, HV = 17.54 ± 0.02; synodic rotational period, Trot = 5.2388 ± 0.0022 hours (the two-peaked solution is assumed); and effective mean diameter, Deff = 1.32 ± 0.06 km. At large phase angles (α ≳ 80○), the polarization phase curve are likely explained by the dominance of large grains and the paucity of small micron-sized grains. We conclude that the polarimetric similarity of these near-Sun asteroids can be attributed to the intense solar heating of carbonaceous materials around their perihelia, where large anhydrous particles with small porosity could be produced by sintering.

(3200) Phaethon polarimetry in the negative branch: new evidence for the anhydrous nature of the DESTINY+ target asteroid

“…We determined 𝑞 inst = −0.05±0.07 %, 𝑢 inst = −0.04±0.11 %, and 𝜃 off = −92.30 ± 0.06 • . These values are in good agreement with previous observations (Ishiguro et al 2022).…”

“…1, the data taken with different instruments agree well, indicating that the data reduction processes described in Section 2 seem to work well to eliminate the instrumental effects. Moreover, Phaethon's profile is in good agreement with (155140) 2005 UD (a dynamical association with Phaethon, Ohtsuka et al 2006), supporting previous results (Ishiguro et al 2022).…”

“…An outline of the data analysis consists of five major steps: (1) preprocessing the raw observed images, (2) extraction of source sig-nals by aperture photometry, (3) correction for systematic errors, (4) derivation of Stokes parameters (𝑞 and 𝑢), polarization degree (𝑃), and polarization position angle (𝜃 P ), and (5) obtaining the nightly weighted mean of 𝑞 and 𝑢. Because we strictly followed the reduction processes (1), ( 2), (4), and (5) written in Ishiguro et al (2022), we skipped the detailed explanation in this paper. The reduction process (3) is particularly important for this work, not only because the polarization degrees at these phase angles are small (𝑃 1 − 2 %) and comparable to the instrumental polarization (an inherent artifact of polarization) of some instruments but also because we need to compare the data taken with different instruments.…”

“…Recently, Ishiguro et al (2022) proposed that polarimetry at low phase angles (Sun-target-observer angle, 𝛼 20 • ) is a useful diagnostic tool for conjecturing if C-complex asteroids are hydrous or anhydrous. However, due to the unfavorable observational conditions until recently, Phaethon's polarimetric property at low phase angles (𝛼 < 19.1 • ) has not been investigated.…”

Evidence of surface heterogeneity on active asteroid (3200) Phaethon

“…This value lies between the grain sizes found to best represent Phaethon's northern and southern areas. Interestingly, the polarization measurements of 2005 UD (P r (α = 100 • ) ≈ 50 − 55%; Ishiguro et al, 2022) most closely match the 2016 polarimetry (Table 6), which are more representative of the Phaethon's northerly areas (Fig. 8).…”