Shape and Rotational Motion Models for Tumbling and Monolithic Asteroid 2012 TC4: High Time Resolution Light Curve with the Tomo-e Gozen Camera

We present visible and near-infrared observations of a near-Earth object (NEO), 2012 TC4. The NEO 2012 TC4 approached close to Earth at a distance of about 50,000 km in 2017 October. This close approach provided a practical exercise for planetary defense. This apparition was also an appropriate opportunity to investigate 2012 TC4, which is a monolithic asteroid. We conducted the observation campaign of 2012 TC4 using six small- and medium-sized telescopes. The multiband photometry analysis showed the taxonomic class of 2012 TC4 to be an X type. In particular, we successfully obtained the high time resolution light curve of 2012 TC4 with the Tomo-e Gozen camera, which is the world's first wide-field CMOS camera, mounted on the 1.05 m Schmidt telescope at Kiso Observatory. The shape and rotational motion models of 2012 TC4 were derived from the light curve. When 2012 TC4 was assumed to be a triaxial ellipsoid, the rotational and precession periods were 8.47 +/- 0.01 minutes and 12.25 +/- 0.01 minutes, respectively, with the long-axis mode. This indicates that 2012 TC4 is a tumbling and monolithic asteroid. The shape models showed the plausible axial lengths to be 6.2 x 8.0 x 14.9 m or 3.3 x 8.0 x 14.3 m. The flattened and elongated shape indicates that 2012 TC4 is a fragment produced by an impact event. We also estimated the excitation timescale, which implied that the impact event happened within similar to 3 x 10(5) yr and 2012 TC4 has a fresh surface.