A study of the physical properties of an active asteroid (6478 Gault)

Abstract

In 2019 January, the appearance of asteroid 6478 Gault immediately attracted attention because this object exhibited a long and thin tail that was quite different from the usual asteroids. This unexpected morphology placed asteroid 6478 Gault into the catalogue of active asteroids. We acquired photometric and spectroscopic observations on 37 nights from 2019 January to April using several telescopes, including LOT (1 m telescope) and SLT (40 cm telescope) at Lulin Observatory, and the 2.4 m telescope at Lijiang station of Yunnan Observatory. We did not find any reliable value for the rotational period of Gault during 2.5 hr and 5 hr observations on 2019 January 26 at Lijiang station and March 25 at Lulin Observatory, respectively. We classified 6478 Gault as a Q-type asteroid using visible spectrum and photometric measurements, including colors (B − Vavg = 0.764 ± 0.045, V − Ravg = 0.450 ± 0.023), and relative reflectance. By using Finson–Probstein analysis, the grain size for Gault’s tail 2 is larger than 20 μm.

Rotational Mass Shedding from Asteroid (6478) Gault

“…This hypothesis, however, could not be confirmed due to the presence of an extensive near-nucleus dust coma, which prevented the detection of a rotational lightcurve in their data. A similar conclusion was also reached by Moreno et al (2019) and Lin et al (2020). On the other hand, Kleyna et al (2019) used high resolution observations from the Hubble Space Telescope to find a period near P ∼ 2 hr, consistent with a rotational instability origin.…”

“…apparition(Moreno et al (2019),Sanchez et al (2019), andLin et al (2020)), but we believe that this was due to the dust emission that existed throughout 2019. The dust velocity was measured at ∼ 0.15 m s −1 (Jewitt et al (2019a)); at this velocity, assuming geocentric distance ∆ = 1.4 AU (as inSanchez et al (2019), it would take ∼8 months for the dust to traverse a 3-arcsec radius photometry aperture.…”

Initial Characterization of Active Transitioning Centaur, P/2019 LD2 (ATLAS), using Hubble, Spitzer, ZTF, Keck, APO and GROWTH Visible & Infrared Imaging and Spectroscopy

“…Although this is roughly consistent with the escape speed of a non-rotating comet nucleus of radius ∼1.8 km as inferred by our observations, it is unlikely that the dust is being ejected exclusively by rotational mass shedding suggested by the low ejection velocity (e.g. Ye et al 2019;Lin et al 2020) and is being transported by the sublimation of volatiles due to the increased activity of the comet as it nears perihelion or a possible combination of these two effects.…”

(6478) Gault: physical characterization of an active main-belt asteroid

“…It is interesting to note that these two classes correspond to the taxonomic type expected for members of the Tamara (C-type) and Phocaea (S-type) families. Moreover, Marsset et al (2019) and Lin et al (2020) both reported a Q-type while the former is observing strong variation of the spectra observed at different epochs. Current space-weathering theories postulate that ordinary chondrite-like asteroids spectroscopically start as Q-types and then gradually turn into S-types due to space-weathering (McFadden et al 1985).…”