Quanzhi Ye (叶泉志)^1,2 et al. (>10)
Astrophysical Journal Letters 874, L16 Link to Article [DOI: 10.3847/2041-8213/ab0f3c]
¹Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
²Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA

Main-belt asteroid (6478) Gault unexpectedly sprouted two tails in late 2018 and early 2019, identifying it as a new active asteroid. Here we present observations obtained by the 1.2 m Zwicky Transient Facility survey telescope that provide detailed time-series coverage of the onset and evolution of Gault’s activity. Gault exhibited two brightening events, with the first one starting on 2018 October 18 ± 5 days and a second one starting on 2018 December 24 ± 1 days. The amounts of mass released are 2 × 10⁷ kg and 1 × 10⁶ kg, respectively. Based on photometric measurements, each event persisted for about a month. Gault’s color has not changed appreciably over time, with a pre-outburst color of g _PS1 − r _PS1 = 0.50 ± 0.04 and g _PS1 − r _PS1 = 0.46 ± 0.04 during the two outbursts. Simulations of dust dynamics shows that the ejecta consists of dust grains of up to 10 μm in size that are ejected at low velocities below regardless of particle sizes. This is consistent with non-sublimation-driven ejection events. The size distribution of the dust exhibits a broken power law, with particles at 10–20 μm following a power law of −2.5 to −3.0, while larger particles follow a steeper slope of −4.0. The derived properties can be explained by either rotational excitation of the nucleus or a merger of a near-contact binary, with the latter scenario to be statistically more likely.