[P-PS04] Small Solar System Bodies: A New Insight from Hayabusa2, OSIRIS-REx and Other Space Missions

[PPS04-22] Highly Time-Resolved Photometric Observations of Tiny Near Earth Objects with the Tomo-e Gozen Camera

*Jin Beniyama¹, Shigeyuki Sako¹, Ryou Ohsawa¹, Satoshi Takita¹, Naoto Kobayashi¹, Shin-ichiro Okumura², Seitaro Urakawa², Fumihiko Usui³, Fumi Yoshida^4,5, Makoto Yoshikawa³ (1.Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2.Japan Spaceguard Association, 3.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 4.University of Occupational and Environmental Health, 5.Planetary Exploration Research Center, Chiba Institute of Technology)

Keywords:asteroid, near earth objects, solar system, light curve, YORP effect

Near Earth Objects (NEO) are solar system bodies whose perihelion distances are smaller than 1.3 au. NEOs are thought to have their origin in the main-belt region between Mars and Jupiter. In addition to the gravitational interaction with the Sun and planets, nongravitational effect which arise from solar radiation plays an important role in their dynamical evolution. YORP effect is one of the nongravitational effects that affects the spin state of an asteroid. YORP effect strongly changes the rotation periods of tiny asteroids (smaller than 100 m in diameter) on a shorter timescale than that of the orbital evolution. Fast rotators suffer from strong centrifugal force and may experience deformation or rotational fission when its rotation period reaches the critical value by YORP spin-up. Therefore the strength of tiny bodies could be inferred using rotation periods of tiny NEOs. Recently, large survey projects have discovered 2,000–3,000 NEOs per year and some fast rotators with rotation periods below a minute have been discovered. Highly time-resolved observations are needed to accurately detect fast rotation because the lower-limit of rotation period which can be detected depends on the overhead time of the instrument. Since the past instruments did not have enought time-resolution, the distribution of rotation periods of NEOs with periods below a minute is still unclear.

We obtained highly time-resolved light curves of 37 tiny NEOs whose diameters are 3–86 m using Tomo-e Gozen camera equipped with 105 cm Schmidt telescope at Kiso Observatory, the University of Tokyo. The observation cadence was 2 fps. We derived the rotation periods of 22 NEOs, of which 5 NEOs show the rotation periods shorter than 20 seconds. The shortest rotation period of our NEOs was about 11 seconds. In spite of high-cadence observations, we found no asteroid rotating shorter than 10 seconds. The lack of tiny fast rotating NEOs has never been reported before our systematic highly time-resolved observation. We discuss the diameter-rotation period relation derived from our observation in terms of YORP spin-up. The observed rotation period distribution is consistent with the senario that the strength of tiny NEOs are weaker than those of meteorites. We also discuss another possible senario that most tiny NEOs are newly-formed in the near-earth region instead of the main-belt.