New Active Asteroid (6478) Gault
Man-To Hui, Yoonyoung Kim, and Xing Gao

TL;DR
This study investigates the dust ejection and activity of asteroid (6478) Gault in 2019, revealing dust properties, ejection speeds, and suggesting rotational instability as the cause of its cometary features.
Contribution
It provides detailed observations of Gault's dust ejection, analyzes dust size distribution and ejection dynamics, and proposes rotational instability as the activity mechanism.
Findings
Dust tails formed in late 2018 and early 2019 with grains 20 μm to 3 mm.
Dust mass declined at 2.28 kg/s, with a temporary surge in March 2019.
No significant non-gravitational acceleration detected in Gault's orbit.
Abstract
Main-belt asteroid (6478) Gault was observed to show cometary features in early 2019. To investigate the cause, we conducted {\it BVR} observations at Xingming Observatory, China, from 2019 January to April. The two tails were formed around 2018 October 26--November 08, and 2018 December 29--2019 January 08, respectively, and consisted of dust grains of 20 m to 3 mm in radius ejected at a speed of m s and following a broken power-law size distribution bending at grain radius 70 m (bulk density 1 g cm assumed). The total mass of dust within a km-radius aperture around Gault declined from kg since 2019 January at a rate of kg s, but temporarily surged around 2019 March 25, because Earth thence crossed the orbital plane of Gault, within which the ejected dust was mainly distributed.…
| Date (UT) | Filter | (s)a | (au)b | (au)c | (°)d | (°)e | (°)f | (°)g | (°)h |
|---|---|---|---|---|---|---|---|---|---|
| 2019 Jan 08 | BVR | 300 | 2.468 | 1.850 | 20.6 | 117.8 | 303.8 | 269.7 | 11.6 |
| 2019 Jan 10 | BVR | 300 | 2.464 | 1.824 | 20.3 | 119.7 | 304.7 | 269.8 | 11.6 |
| 2019 Jan 11 | BVR | 300 | 2.462 | 1.811 | 20.1 | 120.6 | 305.1 | 269.9 | 11.6 |
| 2019 Jan 13 | BVR | 300 | 2.459 | 1.786 | 19.7 | 122.5 | 306.0 | 270.0 | 11.7 |
| 2019 Jan 14 | BVR | 300 | 2.457 | 1.773 | 19.5 | 123.5 | 306.5 | 270.0 | 11.7 |
| 2019 Jan 17 | BVR | 300 | 2.451 | 1.736 | 18.9 | 126.3 | 308.0 | 270.2 | 11.7 |
| 2019 Jan 30 | BVR | 90 | 2.425 | 1.592 | 15.4 | 139.3 | 316.5 | 271.1 | 11.1 |
| 2019 Feb 02 | BVR | 90 | 2.419 | 1.563 | 14.4 | 142.4 | 319.1 | 271.3 | 10.9 |
| 2019 Feb 03 | BVR | 90 | 2.417 | 1.554 | 14.1 | 143.3 | 320.1 | 271.4 | 10.8 |
| 2019 Feb 04 | BVR | 90 | 2.415 | 1.544 | 13.8 | 144.4 | 321.1 | 271.5 | 10.7 |
| 2019 Feb 05 | BVR | 90 | 2.413 | 1.536 | 13.4 | 145.4 | 322.2 | 271.6 | 10.6 |
| 2019 Feb 07 | BVR | 90 | 2.409 | 1.518 | 12.7 | 147.5 | 324.5 | 271.7 | 10.3 |
| 2019 Feb 11 | BVR | 90 | 2.401 | 1.487 | 11.3 | 151.5 | 330.1 | 272.0 | 9.8 |
| 2019 Feb 13 | BVR | 90 | 2.397 | 1.472 | 10.6 | 153.5 | 333.5 | 272.1 | 9.5 |
| 2019 Mar 07 | BVR | 60 | 2.351 | 1.385 | 7.3 | 162.5 | 51.6 | 272.6 | 4.7 |
| 2019 Mar 10 | BVR | 60 | 2.344 | 1.383 | 8.0 | 160.8 | 62.6 | 272.5 | 3.9 |
| 2019 Mar 11 | BVR | 60 | 2.342 | 1.384 | 8.3 | 160.0 | 66.0 | 272.4 | 3.6 |
| 2019 Mar 12 | BVR | 60 | 2.340 | 1.384 | 8.6 | 159.3 | 68.8 | 272.4 | 3.4 |
| 2019 Mar 24 | BVR | 60 | 2.314 | 1.408 | 13.2 | 147.9 | 91.1 | 271.3 | 0.1 |
| 2019 Mar 26 | BVR | 60 | 2.310 | 1.416 | 14.1 | 145.8 | 93.3 | 271.0 | -0.4 |
| 2019 Mar 28 | BVR | 60 | 2.305 | 1.424 | 14.9 | 143.7 | 95.2 | 270.8 | -1.0 |
| 2019 Mar 30 | BVR | 60 | 2.301 | 1.433 | 15.7 | 141.6 | 96.8 | 270.5 | -1.5 |
| 2019 Apr 03 | BVR | 60 | 2.292 | 1.455 | 17.2 | 137.2 | 99.7 | 269.9 | -2.5 |
| 2019 Apr 04 | BVR | 60 | 2.290 | 1.461 | 17.6 | 136.2 | 100.3 | 269.7 | -2.8 |
| Parameter | Value (Tail A) | Value (Tail B) | Comments |
| (m s-1) | N/A | ||
| 0.0 | 0.0 | Fixed value. | |
| 0.0002 | 0.0002 | Accuracy limited by nucleus signal. | |
| 0.035 0.005 | 0.020 0.005 | N/A | |
| Same for both tails assumed. | |||
| (UT) | 2018 Oct 26-Nov 08 | 2018 Dec 29-2019 Jan 08 | N/A |
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New Active Asteroid (6478) Gault
Man-To Hui (許文韜), 1
Yoonyoung Kim (김윤영), 2 and
Xing Gao (高興) 3
1Department of Earth, Planetary and Space Sciences, UCLA, 595 Charles Young Drive East, Los Angeles, CA 90095-1567, USA
2Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, D-37077 Göttingen, Germany
3No. 1 Senior High School of Ürumqi, Ürumqi, Xinjiang, China E-mail: [email protected]
(Accepted XXX. Received YYY; in original form ZZZ)
Abstract
Main-belt asteroid (6478) Gault was observed to show cometary features in early 2019. To investigate the cause, we conducted BVR observations at Xingming Observatory, China, from 2019 January to April. The two tails were formed around 2018 October 26–November 08, and 2018 December 29–2019 January 08, respectively, and consisted of dust grains of 20 µm to 3 mm in radius ejected at a speed of m s*-1* and following a broken power-law size distribution bending at grain radius 70 µm (bulk density 1 g cm*-3* assumed). The total mass of dust within a km-radius aperture around Gault declined from \sim$$9\times 10^{6} kg since 2019 January at a rate of kg s*-1*, but temporarily surged around 2019 March 25, because Earth thence crossed the orbital plane of Gault, within which the ejected dust was mainly distributed. No statistically significant colour or short-term lightcurve variation was seen. Nonetheless we argue that Gault is currently subjected to rotational instability. Using the available astrometry, we did not detect any nongravitational acceleration in the orbital motion of Gault.
keywords:
asteroids: general – asteroids: individual (Gault) – methods: data analysis
††pubyear: 2019††pagerange: New Active Asteroid (6478) Gault – New Active Asteroid (6478) Gault
1 INTRODUCTION
Only recently recognised, active asteroids are a class of solar system small bodies which are indistinguishable from comets observationally but are in dynamically asteroidal orbits (Jupiter Tisserand invariant ; e.g., Jewitt et al., 2015). To date, there are over twenty known members, with a diversity of mass-loss mechanisms including sublimation (e.g., 133P/Elst-Pizarro; Hsieh et al., 2004), rotational instability (e.g., 331P/Gibbs; Drahus et al., 2015), impact (e.g., (596) Scheila; Bodewits et al., 2011; Ishiguro et al., 2011; Jewitt et al., 2011), and thermal fracture (e.g., (3200) Phaethon; Jewitt & Li, 2010; Li & Jewitt, 2013; Hui & Li, 2017). Here we report a discovery of a new member of the class – (6478) Gault (hereafter “Gault").
Gault, formerly designated as 1988 JC1, was discovered at Palomar on 1988 May 12. It has an orbit of semimajor axis au, eccentricity and inclination , leading to a Jupiter Tisserand invariant . Before 2019 there was no published literature on the spectral type and rotation period of Gault whatsoever. In 2019 early January, the Asteroid Terrestrial-Impact Last Alert System (ATLAS) team noticed that the asteroid possessed an obvious narrow tail with high surface intensity, which was absent in previous data taken before early 2018 (Smith et al., 2019), and confirmed by followup observations (e.g., Ye et al., 2019a; Hale et al., 2019). The morphological change has also been monitored by the Zwicky Transient Facility before the discovery by the ATLAS (Ye et al., 2019a, b).
In order to have a better understanding about the mass-loss mechanism at Gault, and the properties of the object itself, we here present photometric and dynamical analysis based upon optical observations from Xingming Observatory.
2 OBSERVATIONS
We conducted observations of Gault using the 0.6 m f/8 Ritchey-Chrétien NEXT (Ningbo Bureau of Education and Xinjiang Observatory Telescope) at Xingming Observatory, Xinjiang, China. Images were taken through the Johnson system B, V, and R filters by a 2k 2k CCD. As the telescope did not track the target nonsidereally, we limited the individual exposure times such that the trailing of Gault in the images did not exceed the typical seeing at Xingming (3″). The images have a pixel scale of 063 pixel*-1* and a square field-of-view (FOV) of 036 036. To maximise the signal from the target we avoided observations in moonlight. The obtained images were bias and dark subtracted and flat-fielded. We summarise our observation information of Gault along with the observing geometry in Table 1. The morphological evolution of Gault is shown in Figure 1.
3 RESULTS
3.1 Photometry
We median combined the nightly images with alignment on Gault and field stars separately, for the sake of better signal-to-noise ratios (SNR). The images with registration on stars had aperture photometric reduction using the Pan-STARRS 1 Data Release 1 (PS1 DR1; Chambers et al., 2016) and system transformation in Tonry et al. (2012) to determine the zero-points. The aperture for stars was 69 (11 pixels) in radius, and the sky flux was computed in annuli having inner and outer radii 104 and 173, respectively. We then conducted aperture photometry of Gault in the coadded images with registration on it using a fixed-size photometric aperture of km in radius. In this step, we computed the sky flux by measuring the flux in neighbouring annuli with inner and outer radii respectively 1.5 and 2 larger than the aperture radius. As we tested, varying the annulus size has negligible effects on the photometry of Gault.
To remove the changing observing geometry, we reduced the apparent magnitude of Gault in bandpass , denoted as , to heliocentric and topocentric distances au and at phase angle using
[TABLE]
where is the compound phase function having the following form:
[TABLE]
Here, is the total flux from both the nucleus and the ejected dust, is the flux from the nucleus, and and are the phase functions of the coma and nucleus, respectively (Hui, 2018). We approximated by the HG formalism (Bowell et al., 1989) with an assumed slope parameter , and by the empirical function by Schleicher & Bair (2011). The absolute magnitudes of the bare nucleus were taken from Ye et al. (2019b) and transformed from the PS1 system to the Johnson system using equations by Tonry et al. (2012). Figures 2 shows the magnitudes of Gault as functions of time. No statistically significant colour variation was seen, mainly because of the dominant errors in the photometric measurements. We obtained the weighted mean values of the colour indices as , , and . Therefore, Gault seems too blue to be a S-type asteroid (Dandy et al., 2003), despite that this class of asteroids is dominant in the Phocaea family (Carvano et al., 2001), to which Gault belongs (Nesvorný, 2015).
We did attempt to investigate the spin period of Gault with the Xingming observations. Photometry was conducted on individual rather than nightly-combined images from 2019 January, because, thanks to the long exposure time, the SNR of the target was the highest. We still failed to discern any repeating short-term variation patterns in the lightcurve above the photometric uncertainty; the lightcurve is essentially flat. Applying the phase dispersion minimisation technique (Stellingwerf, 1978) confirmed that no spin period can be determined, as we found the parameter for periods between 0.5 hr and 1 d ( for correct periods; see Stellingwerf, 1978). Similar to ours, Moreno et al. (2019) also obtained a statistically flat lightcurve of Gault from their independent observations. On the contrary, however, Kleyna et al. (2019) reported the spin period of Gault to be 2 hr. Our failure was possibly caused by the much lower SNR of the target in the uncombined images, the ejected dust around the nucleus, that the nucleus is nearly spherical, or that the line of sight deviated not greatly from the nucleus spin axis.
3.2 Nongravitational Effect
Anisotropic mass loss of Gault may lead to a detectable nongravitational effect because of conservation of angular momentum. To assess this, we utilised the astrometric measurements of Gault111Retrieved from the Minor Planet Center Observation Database (https://minorplanetcenter.net/db_search) on 2019 April 01., which were debiased according to Farnocchia et al. (2015) and weighted based on Vereš et al. (2017), and performed orbit determination with our modified version of the OrbFit package222The original version of the OrbFit package is obtainable from http://adams.dm.unipi.it/~orbmaint/orbfit/.. Perturbations from the eight major planets, Pluto, the Moon, and the most massive 16 asteroids and the relativistic corrections were taken into account. The planetary and lunar ephemerides DE 431 (Folkner et al., 2014) were exploited. The past activity history of Gault is far from clear. Our quick search for the archival observations using the Solar System Object Image Search (Gwyn et al., 2012) revealed that Gault clearly exhibited a tail feature at least in DECam images from 2013 September and 2016 June. We therefore simply assumed the validity of a smooth and symmetric nongravitational force model by Marsden et al. (1973) based on water-ice sublimation. However, as pointed out by Hui & Jewitt (2017) that the isothermal sublimation approximation conflicts with nongravitational effects in Marsden et al. (1973), we instead adopted the hemispherical sublimation model in Hui & Jewitt (2017), whose parameters were obtained from a best fit for a wider heliocentric distance range of au. The six orbital elements along with the radial, transverse and normal (RTN) nongravitational parameters (denoted as , and , respectively; Marsden et al., 1973) of Gault were then treated as free parameters to be solved. Observations with astrometric residuals larger than twice the assigned astrometric uncertainties were discarded (28 out of total 1900 observations with an observing arc from 1984 to 2019), we obtained nondetection (<$$3\sigma) of the nongravitationa force: au d*-2*, au d*-2*, and au d*-2*. This result did not alter significantly if we adopted a stricter or looser outlier rejection criterion, or only a subset of the whole observing arc (e.g., 1999-2019) were used for orbit determination. We thus conclude that, similar to the majority of the active asteroid (Hui & Jewitt, 2017), the mass-loss activity of Gault is not strong enough to exert a detectable nongravitational effect on its orbital motion. The limits to the RTN nongravitational parameters are au d*-2*, au d*-2*, and au d*-2*.
4 DISCUSSION
4.1 Mass Loss
The brightness excess of Gault means a larger effective scattering cross-section than that of a bare nucleus. Assuming that the geometric albedo of the ejected dust and that of the nucleus surface are the same (), and that the optically thin coma is comprised of spherical dust grains of in radius, bulk density g cm*-3*, and following some power-law size distribution , we can estimate the total dust mass within the projected circle around Gault of km in radius from:
[TABLE]
where is the change in the cross-section compared to the effective scattering cross-section of the bare nucleus:
[TABLE]
and is the nucleus radius estimated from the absolute magnitude of the bare nucleus of Gault (Ye et al., 2019b) assuming . The parameters , and were obtained from our morphology analysis (Section 4.2). The result is shown in Figure 3. We can see that, starting from the earliest Xingming observation, the total mass of dust in the aperture continued to decrease from kg until early 2019 March (DOY 70). It indicates the loss of the dust grains within the photometric aperture greater than the supply of newly released counterparts, if any. We obtained the best-fit mean net mass-loss rate during the period between 2019 January 08 and February 13 to be kg s*-1*, which is comparable to some of the known active asteroids such as 133P/(7968) Elst-Pizarro (see Table 2, Jewitt et al., 2015, and citations therein). Interestingly, the object began to brighten starting from DOY 70, peaked around 2019 March 25 (DOY 84), which coincided with the plane-crossing time of Earth, and then declined again. Considering the fact that no new tail corresponding to this brightening was observed afterwards, we prefer that the cause of the brightening in late March was due to the ejected dust grains mainly distributed in the orbital plane of Gault, instead of another outburst event.
4.2 Morphology
The observed morphology of Gault can be used to probe physical properties of the ejected dust grains. The position of an ejected dust grain is known once the release time, initial velocity, and parameter , which is the ratio between the solar radiation pressure acceleration and the local acceleration due to the gravity of the Sun, and also satisfies the relationship , are given. We applied the three-dimensional Monte Carlo dust dynamics model by Ishiguro et al. (2007, 2014) for our morphology analysis. The dust grains were assumed to be ejected isotropically at terminal speeds satisfying the relationship of , where is the velocity of dust grains with and is a constant power index ( for sublimation without cohesion). Based upon our preliminary tests and previous works on non-sublimation-driven active asteroids (e.g., Moreno et al., 2012), we adopted here. The best-fit models (Figure 4) were obtained by comparing the surface brightness profiles of the models and Xingming observations. We found that in order to match the observations, the dust-size distribution has to be a broken power law: for (corresponding to dust-grain radius µm, given the assumed bulk density), and for otherwise. The longer tail (Tail A) was formed at an ejection epoch of 2018 October 26–November 08, while the shorter tail (Tail B) was formed at 2018 December 29–2019 January 08. We used the observations around the time when Earth was nearly in the orbital plane of Gault (see Table 1) to estimate the ejection speed of the dust grains to be m s*-1*. Based on the termination points of the tails, we obtained slightly different values for the two tails. The results are tabulated in Table 2. In general, our conclusion is in good agreement with Ye et al. (2019b).
Similar physical properties of the two tails possibly indicate that they were formed by the same non-sublimation physical process at Gault. Given the non-impulsive durations of the two mass-loss events, plus the fact that Gault was episodically active at least in 2013 and 2016, we argue that the object is in rotational instability due to the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect. The ejection speed of the dust grains m s*-1*, where is the gravitational escape velocity at Gault, along with the YORP spinup timescale shorter than the dynamical timescale of Gault (Kleyna et al., 2019), appear to lend more support on this hypothesis.
5 SUMMARY
We monitored the behaviour of active asteroid (6478) Gault at Xingming Observatory from 2019 January to April. The key conclusions of the analysis are summarised as follows:
Based on our Monte Carlo dust ejection simulation, the two observed tails were formed during two short-lived events that occurred from 2018 October 26 to November 08, and from 2018 December 29 to 2019 January 08, respectively. We infer that the mass-loss activity was caused by rotational instability. 2. 2.
The dust grains were ejected from the nucleus at a common speed of m s*-1* and followed a broken power-law size distribution: for (or µm, assuming g cm*-3*), and for otherwise. 3. 3.
The total mass of dust within the projected radius km from the nucleus generally declined linearly with time from kg since the earliest Xingming observations in early 2019 January at a best-fit rate of kg s*-1*. However, it increased in 2019 March, peaked around March 25, and declined again thereafter, which was due to the fact that most of the dust grains were distributed within the orbital plane of the target. 4. 4.
No statistically significant variations in the short-term lightcurve and colour indices could be detected. The mean colour indices of Gault are , , and . 5. 5.
No nongravitational effect in its orbital motion was detected. We placed limits to the RTN nongravitational parameters as au d*-2*, au d*-2*, and au d*-2*.
Acknowledgements
We thank David Jewitt for comments on the manuscript, and the observers who submitted good astrometric measurements of Gault to the Minor Planet Center.
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