Dust properties of double-tailed active asteroid (6478) Gault

Abstract

Context. Asteroid (6478) Gault was discovered to exhibit a comet-like tail in observations from December 2018, becoming a new member of the so-called active asteroid population in the main asteroid belt. Aims. The aims are to investigate the grain properties of the dust ejected from asteroid (6478) Gault and to give insight into the activity mechanism(s). Methods. We use a Monte Carlo dust tail brightness code to retrieve the dates of dust ejection, the physical properties of the grains, and the total dust mass losses during each event. The code takes into account the brightness contribution of the asteroid itself. The model is applied to a large data set of images spanning the period from January 11, 2019 to March 13, 2019. In addition, both shortand long-term photometric measurements of the asteroid have been carried out. Results. It is shown that, to date, asteroid (6478) Gault has experienced two episodes of impulsive dust ejection, that took place around 2018 November 5 and 2019 January 2, releasing at least 1.4×10 7 kg and 1.6 ×10 6 kg of dust, respectively, at escape speeds. The size distribution, consisting of particles in the 1 µm to 1 cm radius range, follows a broken power-law with bending points near 15 µm and 870 µm. On the other hand, the photometric series indicate a nearly constant magnitude over several 5-7.3 h periods, a possible effect of the masking of a rotational lightcurve by the dust. Conclusions. The dust particles forming Gault's tails were released from the asteroid at escape speeds, but the specific ejection mechanism is unclear until photometry of the dust-free asteroid are conducted, in order to assess whether this was related to rotational disruption or to other possible causes.

Secular and rotational light curves of 6478 Gault

“…The observational orbital velocities are vorb = 1.54, 1.28, 1.15 m/s in very good agreement with the theoretical scape velocity of the dust 0.7 -8 m/s (Chandler et al, 2019;Hui et al, 2019;Jewitt et al, 2019;Kleyna et al, 2019;Moreno et al, 2019;Ye et al, 2019).…”

“…The SLC of 6478 shows evidence of 6 zones of activity, Z1 to Z6. Z1 and Z2 are the outburst studied by (Chandler et al, 2019;Hui et al, 2019;Jewitt et al, 2019;Kleyna et al, 2019;Moreno et al, 2019;Ye et al, 2019).…”

Active Asteroid (6478) Gault: A Blue Q-type Surface below the Dust?

“…(d∼100 µm) derived from morphological analysis of the tails (Jewitt et al 2019;Kleyna et al 2019;Moreno et al 2019;Ye et al 2019b), implying that they do not contain a preponderance of Rayleigh scatterers.…”

“…Bottke et al 2006;Vokrouhlický et al 2015). This remains to be confirmed, as light curves collected so far with various telescopes show very little/no short-term brightness variation (Jewitt et al 2019;Moreno et al 2019;Ye et al 2019b), possibly because the asteroid is embedded in a dust cloud (Licandro et al 2000). Likewise, Ye et al (2019b) pro-posed that activity could be triggered by the merging of a near-contact binary, but currently available light curves do not allow this hypothesis to be validated.…”

A Fireball and Potentially Hazardous Binary Near-Earth Asteroid (164121) 2003 YT1

“…We look into possible dust production mechanisms from 2003 YT 1 . The consequences of YORP-driven breakups are reported from the (sub)km-sized main-belt asteroids, as exampled by P/2010 A2 Agarwal et al 2013), P/2013 R3 (Jewitt et al 2014a(Jewitt et al , 2017Hirabayashi et al 2014) and (6478) Gault (Ye et al 2019;Moreno et al 2019;Kleyna et al 2019;Jewitt et al 2019b;Chandler et al 2019;Hui et al 2019). Additionally, other different mechanisms may work together, e.g.…”