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

Abstract

We present visible and near-infrared observations of a near-Earth object (NEO), 2012 TC 4 . The NEO 2012 TC 4 approached close to the Earth at a distance of about 50,000 km in October 2017. This close approach provided a practical exercise for planetary defense. This apparition was also an appropriate opportunity to investigate 2012 TC 4 , which is a monolithic asteroid (Polishook 2013). We conducted the observation campaign of 2012 TC 4 using six small-and medium-sized telescopes. The multiband photometry analysis showed that the taxonomic class of 2012 TC 4 to be an X-type. In particular, we successfully obtained the high time resolution lightcurve of 2012 TC 4 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 TC 4 were derived from the lightcurve. When 2012 TC 4 was assumed to be a triaxial ellipsoid, the rotational and precession periods were 8.47 ± 0.01 min and 12.25 ± 0.01 min, respectively, with the long axis mode. This indicates that 2012 TC 4 is a tumbling and monolithic asteroid. The shape models showed that the plausible axial lengths to be 6.2 × 8.0 × 14.9 m or 3.3 × 8.0 × 14.3 m. The flattened and elongated shape indicates that 2012 TC 4 is a fragment produced by a impact event. We also estimated the excitation timescale, which implied that the impact event happened within ∼3 × 10 5 yr and 2012 TC 4 has a fresh surface.

Spin Change of Asteroid 2012 TC4 probably by Radiation Torques

“…The X complex contains low and high albedo objects but the E type seems most likely because the high circular polarization ratio suggests that TC4 is optically bright (Reddy et al 2019). This is in agreement with Urakawa et al (2019) who also report X type colors. As a result we used Hapke's model with parameters derived for an E type asteroid (2867) Šteins (Spjuth et al 2012): = 0.57, g = −0.30, h = 0.062, B 0 = 0.6, and θ = 28 • .…”

“…This campaign served also as a planetary defense exercise and its results were summarised by Reddy et al (2019). Additionally, Urakawa et al (2019) also conducted the observing campaign of this asteroid in the same apparition independently and they attempted to reproduce their light curves with a model of a tumbling triaxial ellipsoid. Besides these observing campaigns, a few photometric observations of 2012 TC4 were carried out in this apparition (Sonka et al 2017;Warner 2018;Lin et al 2019).…”