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The Spin-Rate Limits of Small-sized Jupiter Trojans and Hilda Asteroids

Presentation #310.04 in the session Asteroids: Dynamics (Oral Presentation)

Published onOct 23, 2023
The Spin-Rate Limits of Small-sized Jupiter Trojans and Hilda Asteroids

Using the high-cadence lightcurves collected from the FOSSIL survey, Formation of the Outer Solar System: An Icy Legacy – a Subaru/Hyper Suprime-Cam program, conducted in April 2019, May 2020, August 2020, and Octover 2020, rotation periods in a range of ~1 to 10 hours were obtained for 53 small Jupiter Trojans (JTs) and 17 small Hilda asteroids (Hildas). The diameters of the JTs vary from 2 to 40 km, while Hildas range from 1 to 3 km. These are the first reported periods measured for JTs with D < 10 km and Hildas with D ~ 1 km.

The lightcurves were extracted using Hough Transform, an algorithm for line-up detections in the sequential observations, and TRIPPy, a fitting software package for the photometry of the trailed sources. Moreover, the rotation periods were measured using a second-order Fourier series fit.

For JTs, a lower limit of the rotation period of ~4 hr was found, which is shorter than the previously published result of 5 hr found for larger JTs. Assuming a rubble-pile structure for JTs, a bulk density of ~0.9 g cm−3 is required to withstand this spin rate limit, consistent with the value ~0.8–1.0 g cm−3 derived from the binary JT system, (617) Patroclus–Menoetius system.

For Hildas, a spin-rate limit appears at around 3 hr. Assuming rubble-pile structures for the Hildas, a bulk density of ~1.5 g cm−3 is required to withstand this spin-rate limit. This value is similar to that of the C-type asteroids (1.33 g cm−3) and higher than the ~1 g cm−3 of the JTs. This result suggests that Hildas may contain more C-type asteroids than expected, and the limit at 3 hr simply reflects the spin rate limit for C-type asteroids. In addition, a superfast rotator was found for Hildas, with a rotation period of 1.633 hr and an estimated diameter of 0.7 km. This object is unlikely to be explained by a rubble-pile or monolithic structure.

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