Episodically Active Asteroid 6478 Gault

David Jewitt1,2, Yoonyoung Kim3, Jane Luu4, Jayadev Rajagopal5, Ralf Kotulla6, Susan Ridgway5, and Wilson Liu5
Astrophysical Journal Letters 876, L19 Link to Article [DOI: 10.3847/2041-8213/ab1be8]
1Department of Earth, Planetary and Space Sciences, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095-1567, USA
2Dept. of Physics and Astronomy, UCLA, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA
3Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
4Department of Physics and Technology, Arctic University of Tromso, Tromso, Norway
5NOAO, 950 North Cherry Avenue, Tucson, AZ 85719, USA
6Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA

We present imaging and spectroscopic observations of 6478 Gault, a ~6 km diameter inner main-belt asteroid currently exhibiting strong, comet-like characteristics. Three distinct tails indicate that ultra-slow dust (ejection speed 0.15 ± 0.05 m s−1) was emitted from Gault in separate episodes beginning UT 2018 October 28 ± 5 (Tail A), UT 2018 December 31 ± 5 (Tail B), and UT 2019 February 10 ± 7 (Tail C), with durations of ΔT ~ 10–20 days. With a mean particle radius $\overline{a}\,\sim $ 200 μm, the estimated masses of the tails are M A  ~ 4 × 107 kg, M B  ~ 6 × 106 kg, and M C  ~ 6 × 105 kg, respectively, and the mass-loss rates from the nucleus are 20–40 kg s−1 for Tail A, 4–6 kg s−1 for Tail B, and ~0.4 kg s−1for Tail C. In its optical colors Gault is more similar to C-type asteroids than to S-types, even though the latter are numerically dominant in the inner asteroid belt. A spectroscopic upper limit to the production of gas is set at 1 kg s−1. Discrete emission in three protracted episodes effectively rules out an impact origin for the observed activity. Sublimation driven activity is unlikely given the inner-belt orbit and the absence of detectable gas. In any case, sublimation would not easily account for the observed multiple ejections. The closest similarity is between Gault and active asteroid 311P/(2013 P5), an object showing repeated but aperiodic ejections of dust over a 9 month period. While Gault is 10 times larger than 311P/(2013 P5), and the relevant timescale for spin-up by radiation torques is ~100 times longer, its properties are likewise most consistent with episodic emission from a body rotating near breakup.

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