New Insights into the Active Asteroid (6478) Gault

The discovery of outgassing activity by the main-belt asteroid (6478) Gault in 2018 inspired AO Postdoctoral Researcher Maxime Devogéle to lead a study using photometric and spectroscopic observations of the asteroid and archival photographic plate surveys. It also included previously unknown detections of Gault to characterize the asteroid’s composition, rotation period, and orbital changes, and to monitor its outburst activity.

TITLE
New Insights into the Active Asteroid (6478) Gault

INVESTIGATORS
Maxime Devogèle, Marin Ferrais, Emmanuel Jehin, Nicholas Moskovitz, Brian A Skiff, Stephen E Levine, Annika Gustafsson, Davide Farnocchia, Marco Micheli, Colin Snodgrass, Galin Borisov, Jean Manfroid, Youssef Moulane, Zouhair Benkhaldoun, Artem Burdanov, Francisco J Pozuelos, Michael Gillon, Julien de Wit, Simon F Green, Philippe Bendjoya, Jean-Pierre Rivet, Luy Abe, David Vernet, Colin Orion Chandler, Chadwick A Trujillo

ABSTRACT
In 2018 December, the main-belt asteroid (6478) Gault was reported to display activity. Gault is an asteroid belonging to the Phocaea dynamical family and was not previously known to be active, nor was any other member of the Phocaea family. In this work, we present the results of photometric and spectroscopic observations that commenced soon after the discovery of activity. We obtained observations over two apparitions to monitor its activity, rotation period, composition, and possible non-gravitational orbital evolution. We find that Gault has a rotation period of P = 2.4929 ± 0.0003 h with a light-curve amplitude of 0.06 magnitude. This short rotation period close to the spin barrier limit is consistent with Gault having a density no smaller than ρ = 1.85 g cm−3 and its activity being triggered by the YORP (Yarkovsky–O’Keefe–Radzievskii–Paddack) spin-up mechanism. Analysis of the Gault phase curve over phase angles ranging from 0.4° to 23.6° provides an absolute magnitude of H = 14.81 ± 0.04, G1 = 0.25 ± 0.07, and G2 = 0.38 ± 0.04. Model fits to the phase curve find the surface regolith grain size constrained between 100 and 500 μm. Using relations between the phase curve and albedo, we determine that the geometrical albedo of Gault is pv = 0.26 ± 0.05 corresponding to an equivalent diameter of D=2.8+0.4−0.2 km. Our spectroscopic observations are all consistent with an ordinary chondrite-like composition (S, or Q-type in the Bus-DeMeo taxonomic classification). A search through archival photographic plate surveys found previously unidentified detections of Gault dating back to 1957 and 1958. Only the latter had been digitized, which we measured to nearly double the observation arc of Gault. Finally, we did not find any signal of activity during the 2020 apparition or non-gravitational effects on its orbit.
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Keywords: arecibo, observatory, gault, Phocaea, Devogéle, orion, Bus-DeMeo, asteroid, radio, telescope