Time-series and Phasecurve Photometry of Episodically-Active Asteroid (6478) Gault in a Quiescent State Using APO, GROWTH, P200 and ZTF
Josiah N. Purdum, Zhong-Yi Lin, Bryce T. Bolin, Kritti Sharma, Philip I. Choi, Varun Bhalerao, Harsh Kumar, Robert Quimby, Joannes C. Van Roestel, Chengxing Zhai, Yanga R. Fernandez, Josef Hanuš, Carey M. Lisse, Dennis Bodewits, Christoffer Fremling, Nathan Ryan Golovich, Chen-Yen Hsu, Wing-Huen Ip, Chow-Choong Ngeow, Navtej S. Saini, Michael Shao, Yuhan Yao, Tomás Ahumada, Shreya Anand, Igor Andreoni, Kevin B. Burdge, Rick Burruss, Chan-Kao Chang, Chris M. Copperwheat, Michael Coughlin, Kishalay De, Richard Dekany, Alexandre Delacroix, Andrew Drake, Dmitry Duev, Matthew Graham, David Hale, Erik C. Kool, Mansi M. Kasliwal, Iva S. Kostadinova, Shrinivas R. Kulkarni, Russ R. Laher, Ashish Mahabal, Frank J. Masci, Przemyslaw J. Mróz, James D. Neill, Reed Riddle, Hector Rodriguez, Roger M. Smith, Richard Walters, Lin Yan, Jeffry Zolkower
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Time-series and Phasecurve Photometry of Episodically-Active Asteroid (6478) Gault in a Quiescent State UsingAPO, GROWTH, P200 and ZTF
Josiah N. Purdum, Zhong-Yi Lin ∗ , Bryce T. Bolin ∗ , Kritti Sharma, Phillip I. Choi, Varun Bhalerao, Harsh Kumar,
6, 7
Robert Quimby,
1, 8
Joannes C. Van Roestel, Chengxing Zhai, Yanga R. Fernandez, Josef Hanuˇs, Carey M. Lisse, Dennis Bodewits, Christoffer Fremling, Nathan Ryan Golovich, Chen-Yen Hsu, Wing-Huen Ip, Chow-Choong Ngeow, Navtej S. Saini, Michael Shao, Yuhan Yao, Tom´as Ahumada, Shreya Anand, Igor Andreoni, Kevin B. Burdge, Rick Burruss, Chan-Kao Chang, Chris M. Copperwheat, Michael Coughlin, Kishalay De, Richard Dekany, Alexandre Delacroix, Andrew Drake, Dmitry Duev, Matthew Graham, David Hale, Erik C. Kool,
23, 24
Mansi M. Kasliwal, Iva S. Kostadinova, Shrinivas R. Kulkarni, Russ R. Laher, Ashish Mahabal,
9, 26
Frank J. Masci, Przemyslaw J. Mr´oz, James D. Neill, Reed Riddle, Hector Rodriguez, Roger M. Smith, Richard Walters, Lin Yan, and Jeffry Zolkower Department of Astronomy, San Diego State University, 5500 Campanile Dr, San Diego, CA 92182, U.S.A. Institute of Astronomy, National Central University, Taoyuan 32001, Taiwan ∗ IPAC, Mail Code 100-22, Caltech, 1200 E. California Blvd., Pasadena, CA 91125, U.S.A. ∗ Department of Mechanical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India Physics and Astronomy Department, Pamona College, 333 N. College Way, Claremont, CA 91711, U.S.A. Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India LSSTC Data Science Fellow Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, TheUniversity of Tokyo, Kashiwa, Chiba 277-8583, Japan Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, U.S.A. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, U.S.A. Dept. of Physics and Florida Space Inst., Univ. of Central Florida, 4000 Central Florida Boulevard, Orlando FL 32816-2385, USA Institute of Astronomy, Faculty of Mathematics and Physics, Charles University, V Holeˇsoviˇck´ach 2, 18000 Prague, Czech Republic Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723 Physics Department, Leach Science Center, Auburn University, Auburn, AL 36832, U.S.A. Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, U.S.A. Graduate Institute of Astronomy, National Central University, 32001, Taiwan Institute of Astronomy, National Central University, 32001, Taiwan Department of Astronomy, University of Maryland, College Park, MD 20742, U.S.A. Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, U.S.A. Astrophysics Research Institute Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, United Kingdom School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA DivisionThe Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691, Stockholm, Sweden Department of Physics and Astronomy, Macquarie University, NSW 2109, Sydney, Australia IPAC, Mail Code 100-22, Caltech, 1200 E. California Blvd., Pasadena, CA 91125, USA Center for Data Driven Discovery, California Institute of Technology, Pasadena, CA 91125, U.S.A.
ABSTRACTWe observed Episodically Active Asteroid (6478) Gault in 2020 with multiple telescopes in Asia andNorth America and have found that it is no longer active after its recent outbursts at the end of 2018and start of 2019. The inactivity during this apparation allowed us to measure the absolute magnitudeof Gault of H r = 14.63 ± G r = 0.21 ± Corresponding author: Josiah [email protected] a r X i v : . [ a s t r o - ph . E P ] F e b Purdum et al. repeating (cid:46) ∼ ∼ INTRODUCTIONActive asteroids produce comet-like tails and comae that can be driven by many different types of forces differentfrom comets themselves (Jewitt et al. 2015). While sublimation of water ice is a primary driver for activity in ‘typical’comets, the ∼
20 known (so far) active asteroids in the Main Belt seem to lose mass via a wider array of physical effectssuch as collisions (e.g., Snodgrass et al. 2010), rotational instabilities (e.g., Jewitt et al. 2013), and thermal fracture(e.g., Jewitt et al. 2019a). We can assess the physics of a particular active asteroid’s activity via observations overlong time baselines that assess the object’s photometric and morphological development. As more and more activeasteroids are discovered, it is vital to continuously monitor these objects and determine the frequency of the variousphenomena in the Main Belt.Main Belt asteroid (6478) Gault (1998 JC ; “Gault” hereafter) has been the subject of wide interest since thediscovery in early 2019 of comet tail-like extended emission (Smith et al. 2019). Eventually three tails were notedin January 2019 (Ye et al. 2019; Jewitt et al. 2019b) on the S-type Phocaea family member (Sanchez et al. 2019),suggesting multiple sporadic outbursts of activity. Ye et al. (2019) assessed the dynamics of the dust seen nearGault and estimated that two outbursts had actually occurred in late 2018. Searches through archival data thatserendipitously caught Gault revealed that there had been active episodes in 2013, 2016, and 2017 as well (Chandleret al. 2019).Many authors have proposed that the cause of Gault’s activity is the instability of material on its surface (Huiet al. 2019; Jewitt et al. 2019b; Kleyna et al. 2019; Moreno, F. et al. 2019; Ye et al. 2019) due its rotation beingspun up by the the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect (Bottke Jr et al. 2006; Kleyna et al. 2019).A critical observational test of this hypothesis would be to measure the asteroid’s rotational period. Unfortunately,due to the dust around the asteroid, the rotational period had not been well constrained as reflected sunlight fromdust grains would swamp the signal from the asteroid itself and thus suppress short-term lightcurve variations due toGault’s shape, as indeed Jewitt et al. (2019b) concluded. However there have been published reports of some hints ofrotational signatures in lightcurve data. For example, Ivanova et al. (2020) suggest a rotation period of 1.79 hours,Carbognani & Buzzoni (2020) suggest 3.34 hours, Ferr´ın et al. (2019) have 3.36 hours, and Kleyna et al. (2019) suggest ∼ § § § OBSERVATIONSFor our analysis we have made use of both our own, PI-led, pointed observations using several telescope facilities inthe GROWTH (Global Relay of Observatories Watching Transients Happen) network (Kasliwal et al. 2019) and otherfaculties, as well as archival data from the Zwicky Transient Survey (ZTF, Graham et al. 2019; Bellm et al. 2019).Our pointed observations occurred on 17 nights between 2019 January 8 and 2020 October 20, and made use of sixtelescopes. On 7 of those 17 nights, we were able to have multiple telescopes follow Gault in a coordinated effort. Theobservations in 2019 showed Gault to still be active, but all such observations in 2020 showed no activity, only a point ∗ These authors contributed equally to this work. hotometric Lightcurves of inactive (6478) Gault
Mount Laguna Observatory 1.0-Meter Telescope
Images of Gault were taken with the Mount Laguna Observatory (MLO) 1.0-meter telescope on 2020 June 24 UT,several months after the asteroid was leaving Solar conjunction. 120 images were taken with 30-second exposure timeseach, culminating 60 minutes of total exposure to measure the morphology of the asteroid.Later observations were taken of Gault with MLO with an aim to constrain a rotation period. This data was takenbetween 2020 August 23 UT and 2020 October 20 UT over 6 separate campaigns listed in Table 2. MLO lightcurveimages were taken in the Johnson-Cousins R filter with between 165 and 190 separate 120-second exposures.2.2.
Lulin One-meter Telescope
The time-series observations of Gault using the Lulin one-meter Telescope (LOT) at the Lulin Observatory, Taiwan,for 2020 August 23 and 24 UT lasted 6.4 hours and 5.2 hours, respectively. The other time span on 2020 September21 UT and 2020 October 11 UT lasted 6.4 hours and 6.8 hours, respectively. Except for the use of unfiltered CCDobservations early in the campaign, all observations are acquired with R-filter and obtained through non-siderealtracking. 2.3.
Astrophysics Research Consortium 3.5-meter Telescope
Between 2019 January 8 UT and 2019 June 6 UT we observed Gault over 6 campaigns with the AstrophysicsResearch Consortium 3.5-meter Telescope (ARC) at Apache Point Observatory before the asteroid headed into Solarconjunction (Table 2). Individual observations created lightcurves spanning between 1 hour (2019 March 24 UT) and4 hours (2019 April 26 UT). Observations were taken with the ARCTIC optical CCD (Huehnerhoff et al. 2016) inthe r (cid:48) filter with an average seeing of 1.8”. Throughout January and February of 2019, Gault remained visibly active,while observations between March and June of 2019 displayed just a remnant tail (see Figures 1 and 5).2.4. Palomar Observatory 200-Inch Telescope
On 2020 Aug 27 UT, the Palomar Observatory 200-inch telescope (P200) observed Gault in the r band. The 103exposures each had an equivalent exposure time of 90 seconds, accumulating in a total of 9,270 seconds with an averageseeing of ∼ r band in this work.2.5. Zwicky Transient Facility using the Palomar 48-Inch Telescope
Images of Gault were taken with the Zwicky Transient Facility (ZTF) (Graham et al. 2019) which is mounted onthe Palomar Observatory’s 48-inch telescope (P48) (Bellm et al. 2019; Dekany et al. 2020). ZTF images are locatedin the ZTF archive (Masci et al. 2019) and Gault’s photometry was measured with a 5” aperture and processed usingthe ZChecker software (Kelley et al. 2019). The 30-second exposure time observations were made in the r band andcolor-corrected using the g − r value of 0.50 ± GROWTH-India Telescope
We observed Gault on multiple nights with the 0.7m GROWTH-India telescope (GIT), using the SDSS r (cid:48) filter andan Apogee KAF3200EB camera giving a ∼ (cid:48) × . (cid:48) field of view. Due to the slow motion of Gault, we used siderealtracking and took multiple exposures. Data were acquired in 120 sec exposures on 2020 September 21, followed by180 sec exposures on 2020 October 16 and October 20. Data were downloaded and processed in real time at our dataprocessing machine at IIT Bombay. We calibrated images for processing by applying bias correction and flat-fielding,obtained astrometry solution using offline engine of astrometry.net (Lang et al. 2010) and finally removing cosmic-rays Purdum et al.
Figure 1.
Left : 4680-second Deep-stack image of Gault from the ARC 3.5-meter telescope at Apache Point Observatory on2019 February 25 UT. The two tails indicate two separate epochs of activity.
Right : 4320-second Deep-stack image of Gaultfrom the ARC 3.5-meter on 2019 April 26 UT after it had produced a third tail. via Astro-SCRAPPY (McCully & Tewes 2019) package. photometry was performed using PyRAF based processingpipeline. We cross-matched the the sextractor-identified (Bertin 2011) sources in the GIT image with
GROWTH Coordinated Observations
On 2020 September 21 UT, LOT, GIT, MLO, and the Table Mountain Observatory (TMO) 1.0-meter telescopeparticipated in a 20-hour relay of observing Gault for photometric lightcurve variation. MLO started the relay at2020-09-21 04:24:47 UT (airmass 2.0) and ran continuous observations of 120-second exposure times until 11:35:27 UT(airmass 2.0) the same day, totaling 22800 seconds of exposure time. LOT took over shortly after at 14:26:07 (airmass1.2) amassing 14130 seconds of 90-second exposures before finishing at 20:29:09 UT (airmass 2.5). GIT observed Gaultfor roughly 6 hours in 135 images amassing 16,200 seconds of exposure starting from 17:21:31 UT (airmass 1.2), inthe middle of the LOT observations, and ending at 23:18:07 UT (airmass 2.5). TMO observed Gault for roughly4 hours in 225 images amassing 13,500 seconds of exposure starting at 06:36:37 UT (airmass 1.3), in the middle ofMLO’s observations. Typical seeing for GIT is 2.5” and for TMO is 1.5”. We were able to take data with TableMountain Observatory despite the degraded conditions caused by a nearby wildfire. Additional coordination amongthe GROWTH network include MLO-LOT observations on 2020 August 23 and 24 UT and 2020 October 11 UT,TMO-LOT on 2020 September 22 UT, and MLO-GIT on 2020 October 16 and 20 UT. RESULTS3.1.
Active and Inactive states of Gault
Follow-up observations with the ARC 3.5-meter telescope at Apache Point Observatory on 2019 February 25 UTshowed evidence for multiple tails. A deep-stack image consisting of 4680 seconds of exposure is shown on the left sideof Figure 1. The surface brightness of Gault in this stack is 24.0 mag/sq.arcsec within a 10,000 km aperture. Theright side of Figure 1 shows Gault’s third tail in a 4320-second deep-stack image from the ARC 3.5-meter telescopeon 2019 April 26 UT and the 10,000 km surface brightness was calculated to be 23.8 mag/sq.arcsec. The images werecombined in deep, median-stacks centered on Gault and then used to compute the calibrated r -band photometry fromcomparisons to a similar deep-stacked images of reference stars with Solar colors from the same initial image. Wereferenced photometry of the reference star from the Pan-STARRS catalog (Chambers et al. 2016).Both surface brightnesses were brighter than the surface brightness of 25.8 mag/sq.arcsec found when it was inactivein 2020 June by Purdum et al. (2020). Our deep-stack image taken by the P200 in the r band is shown in Figure2, which lacks cometary features and has a surface brightness of 26.3 mag/sq.arcsec, also dimmer than the surfacebrightnesses from early 2019. 3.2. Secular Photometry and Updated Absolute Magnitude
Figure 3 shows Gault’s reduced magnitude phasecurve from ZTF data taken between 2017 June 14 UT to 2017December 2 UT. The reduced magnitude data is described by r − ( r ∆) where r and ∆ are the heliocentric and hotometric Lightcurves of inactive (6478) Gault Figure 2.
Deep-stack image of Gault taken with the P200 telescope in 103 90-second images culminating in 9270-seconds on2020 August 27 UT in the r band. The image displays a lack of coma or southwest-facing tail implying the inactivity of Gault. geocentric distances. We can then find the r -band absolute magnitude H r and phase slope parameter G by fitting thereduced magnitudes to the phase function of the form: r − ( r ∆) = H − . [(1 − G ) Φ ( α ) + G Φ ( α )] (1)where r is the r -band magnitude of Gault taken with a 5 (cid:48)(cid:48) aperture, α is the phase angle of the asteroid at the timeof mid-exposure, and Φ and Φ are two basis functions normalized at unity for α = 0 ◦ (Bowell et al. 1989; Muinonenet al. 2010; Pravec et al. 2012).These best fit parameters are H r = 14.631 ± G r = 0.207 ± ± r band ZTF photometry between June and October of 2020. Gault exited Solarconjunction and was observed between 2020 April 02 UT (MJD = 58941) and 2020 October 14 UT (MJD = 59136).Comparing the measured equivalent r-band magnitude of Gault from photometry measured in ZTF observationstaken on these dates with the predicted magnitude of Gault based on our measured r-band absolute magnitude of H r = 14 . ± .
019 and phase function slope value G = 0 . ± . Time-Series Lightcurves
While Gault was active in 2019, the ARC 3.5-meter telescope at Apache Point Observatory took short-periodlightcurve images, as shown in Figure 5. Much like Figure 6 in Jewitt et al. (2019b), this plot shows little variation inthe lightcurve while the asteroid was experiencing activity and producing comet-like features. The variations that dooccur in these lightcurves have small-amplitude peaks and are caused by noise consistent with the uncertainty valuesin their individual differential photometry.
Purdum et al.
Figure 3.
Reduced magnitude of Gault from Equation 1 as a function of phase angle. The data points are taken from P48observations with ZTF starting 2017 June 14 UT and ending 2017 December 2 UT. The line of best fit is based on r − ( r ∆)in Equation 1 using H r = 14 . ± .
019 and G = 0 . ± . Figure 4.
Secular photometry of Gault from 2018 November 01 UT (MJD = 58423) to 2020 October 14 UT (MJD = 59136).The red points are the apparent magnitudes of Gault taken by the Palomar Observatory 48-inch telescope and ZTF in the r band over this time span. Data presented from before 2019 February 10 UTC are adapted from Ye et al. (2019). The black line isthe predicted apparent V-band magnitude from JPL HORIZON’s ephemeris service. The blue line indicates what the predictedmagnitude should be given new values for the absolute magnitude H = 14 . ± .
019 and slope parameter G = 0 . ± . The flatness of the lightcurves is noted even as the brightness of Gault began to resemble its predicted brightnessbased on its pre-activity H r seen in Figure 4. In order to determine the rotational period of Gault after it returned toan inactive state, we obtained coordinated long-term lightcurves on nine separate dates starting in August 2020, withthe longest single lightcurve of 19 hours on 2020 September 21 UT. The results are displayed in Figures 6 and 7. hotometric Lightcurves of inactive (6478) Gault Figure 5.
The separate observatories are indicated by color, with MLO as red, LOT as green, P200 as orange, TMO as purple,and GIT as blue. Each lightcurve for each observatory has an additional reference star lightcurve normalized to -0.3magnitude to show that the reference stars used to calculate the differential photometry of Gault did not vary overtime (or if so, not at the same period and amplitude as the asteroid’s lightcurves). The reference star lightcurves havesmall error bars and have not been corrected for airmass or weather effects on their photometry and therefore arecurved, but display no strong signs of periodicity.Each date in Figures 6 and 7 also contain vertical orange lines indicate small-amplitude, periodic peaks in theasteroid lightcurves based on 2.49-hour intervals from an arbitrary t = 0 denoted by the green line in Figure 6. Thepeaks roughly halfway between the orange lines are denoted with a vertical light-blue line. The peaks repeat roughlyevery 1.25 - 2.5 hours, indicating a possible rotation period around 2.5 hours.A Lomb-Scargle (Lomb 1976) periodogram was constructed from MLO and LOT data of Gault ranging from Augustto October of 2020 and is shown along with a folded lightcurve of Gault in Figure 8. The differential photometrytechnique we used to create the periodogram relied on determining the difference between the brightness of Gault andthe comparison stars in the same field of view to acquire the Gault’s light curve. The comparison stars we selecteddepended on the maximum frame width that we used through one run and on the similarity of FWHM (Full Widthat Half Maximum) estimated between Gault and the chosen stars. By comparing the reference stars’ light curves,some variable stars were ruled out in the photometric analysis. To combine several photometry runs through differentnights, the mean values of each run were automatically scaled using an IDL routine that we created. We then searchedfor significant periodicities using the Lomb-Scargle periodogram functions on the combined light curve data to findthe most likely rotation period of Gault. The frequency analysis from the strong peak near ∼
20 cycles/day in Figure8 gives a rotation period of ∼ ± ± .
07 hours is estimated using the bootstrap method (Press et al. 1986) which removed √ N data points fromthe time series lightcurves and recalculated the period value from the Lomb-Scargle periodogram. This process wasrepeated 10,000 separate times with the resulting central value of 2.49 hours and a 1 − σ uncertainty estimate of 0.07hours. Purdum et al.
Figure 6.
Coordinated long-term photometric lightcurves of Gault and various reference stars (offset -0.3mag) starting from2020 August 14 UT and organized chronologically to 2020 September 21 UT. The observatories are color-coordinated and therotation period of 2.49-hours is denoted with orange vertical lines near some peaks in the lightcurve which are based on thearbitrary green t = 0 line on 2020 September 21 UT. Additional peaks are marked with light-blue lines are split equally betweenthe orange lines. The lightcurves continue through 2020 October 20 UT in Figure 7. hotometric Lightcurves of inactive (6478) Gault Figure 7.
Coordinated long-term photometric lightcurves of Gault and various reference stars (offset -0.3mag) starting from2020 September 22 UT and organized chronologically to 2020 October 20 UT. This figure is a continuation of Figure 6 wherethe observatories are color-coordinated and the rotation period of 2.49-hours is denoted with orange vertical lines near somepeaks in the lightcurve which are based on the arbitrary green t = 0 line on 2020 September 21 UT. The light-blue vertical linesmark additional peaks that split the orange lines. Purdum et al.
Figure 8.
Left : Lomb-Scargle Periodogram of Gault’s lightcurve data from MLO and LOT observations starting 2020 August23 UT and ending 2020 October 20 UT.
Right : Folded lightcurve of Gault from MLO and LOT with a period of 2.5 hoursaveraged over 10 invidual MLO and LOT ligthcurves taken over six nights of data taken between 2020 August 23 UT and ending2020 October 20 UT.
The amplitude of the MLO and LOT lightcurves are low, so it is somewhat difficult to recognize a continuousvariation in brightness in the folded version of the lightcurve, as seen on the right in Figure 8. DISCUSSION AND SUMMARYThe surface brightness profiles taken from deep-stacked images of Gault from the MLO 1.0-meter on 2020 July 21UT (Purdum et al. 2020) and P200 on 2020 August 27 UT indicate that Gault is no longer active after it appearedto have an outburst of material that caused multiple tails to form starting in October of 2018 (Ye et al. 2019). Thesurface brightness measurements of 25.8 mag/sq.arcsec and 26.3 mag/sq.arcsec, from MLO and Palomar, respectively,are fainter compared to the surface brightness values of Gault from the ARC 3.5-meter Telescope on 2019 February9 UT, had a measurement of 24 mag/sq.arcsec (Purdum et al. 2020). The fainter measurements in 2020 could meanthat Gault no longer has material surrounding it and can be deemed inactive.Gault’s deactivation can also be seen over time in the photometry from ZTF observations of Gault in Figure 4. Theactivity of Gault is apparent on the left side of the plot with the data being much brighter and more variable thanthe predicted magnitudes from JPL’s HORIZONS ephemeris service . The outbursts for Gault’s first two tails wereestimated to have occurred on 2018 October 18 ± ± ± G in Equation 1 from 0.25 to 0.21 and theabsolute magnitude H from JPL’s HORIZONS’ 14.3 to 14.6, re-aligns the photometry in Figure 4. This phenomenoncan be attributed to the observing geometry of Gault changing throughout its orbit. The higher absolute magnitude H r in 2019 could be due to pole-on observations, while the equator-on observations would result in a smaller absolutemagnitude in 2020.Photometric lightcurve observations with the ARC 3.5-meter while Gault was still dust-dominated (see Figure 5) in2019 started to show some variation as the activity on Gault diminished, but the low-amplitude of the variations werenot enough to conclude a rotation period. Our observations of Gault in 2020 also produced low-amplitude lightcurves https://ssd.jpl.nasa.gov/?horizons hotometric Lightcurves of inactive (6478) Gault ∼ ± ∼
20 cycles/day ( ∼ b / a axial ratio is close to 1–1.3 given its maximum possiblelightcurve amplitude of ∼ b / a and lightcurve amplitude of b / a = 10 . M from Binzel et al. (1989). The critical breakup period of a strengthlessellipsoid as a function of axial ratio is given by Jewitt et al. (2017) as P crit = (cid:18) ba (cid:19)(cid:20) πGρ (cid:21) / (2)where ρ is the density of the ellipsoid and G is the Newtonian gravitational constant. Gault should have a density ofroughly equal to 2.2 g/cm (Sanchez et al. 2019; Marsset et al. 2019), consistent with other S-Type asteroids (Carry2012). Figure 9 presents the critical period with which Gault would start shedding surface material as a functionof the axial ratio and density. An orange square labelled in Figure 9 is likely to contain the critical rotation periodfor an object with Gault’s geometry. A rotation period like the one we have found at 2.5 hours seems to be at ornear the critical period of Gault and therefore could be the cause of the activity started in 2018. Previous authorshave also proposed Gault’s activity was caused by rotational instability induced from the YORP effect (Jewitt et al.2019b; Kleyna et al. 2019; Ferr´ın et al. 2019). Jewitt et al. (2015) determined that the YORP spin-up timescale forGault should be roughly 22 Myr, much shorter than the 100 Myr-timescale for the re-orientation of the spin of a ∼ Purdum et al.
Figure 9.
Adapted from Bolin et al. (2018). The critical rotation period of an asteroid based on the axial ratio b/a and thedensity ρ . The likely critical period for Gault is indicated in the orange box, roughly around 2.0 - 3.0 hours. Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the PalomarObservatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundationunder Grant No. AST-2034437 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science,the Oskar Klein Center at Stockholm University, the University of Maryland, Deutsches Elektronen-Synchrotron andHumboldt University, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, Trinity CollegeDublin, Lawrence Livermore National Laboratories, and IN2P3, France. Operations are conducted by COO, IPAC,and UW.This work was supported by the GROWTH project funded by the National Science Foundation under PIRE GrantNo 1545949.Part of this work was performed under the auspices of the U.S. Department of Energy by Lawrence LivermoreNational Laboratory under Contract DE-AC52-07NA27344.B.T.B. and F.J.M. acknowledge support from NASA with grant number 80NSSC19K0780.C.F. gratefully acknowledges the support of his research by the Heising-Simons Foundation ( hotometric Lightcurves of inactive (6478) Gault
Facility:
Apache Point Astrophysical Research Consortium 3.5 m telescope, GROWTH India Telescope, LulinOptical Telescope, Mount Laguna Observatory 40-inch Telescope, P48 Oschin Schmidt telescope/Zwicky TransientFacility, Table Mountain Observatory
Software:
Astropy, ZChecker, Aperture Photometry ToolREFERENCES
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Table 1 . Telescope Specifications and Parameters for This WorkTelescope CCD Pixels Binning Scale (”/pix) Exp (s) NST MLO 1.0-meter ULTRAcam 2Kx2K 2x2 0.358 120 NLulin One-meter SOPHIA 2Kx2K 1x1 0.385 180 YARC 3.5-meter ARCTIC 2Kx2K 2x2 0.228 120 YPalomar 200-inch CHIMERA 1Kx1K 1x1 0.29 90 YPalomar 48-inch ZTF CCD 16 6Kx6K 1x1 1.01 30 NGIT 0.7-meter Apogee KAF3200EB 2Kx1K 1x1 0.3 120-180 NTMO 1.0-meter sCMOS 1.6Kx1.6K 1x1 0.225 60 N
Table 1 . Columns: (1) Telescope name; (2) CCD Camera name; (3)Number of Pixels on CCD; (4) Type of binning used; (5) Pixel scale; (6)Exposure Time; (7) Non-sidereal tracking enabled (Y/N).
Table 2 . Observations of Gault producing photometric lightcurves.Date (UTC) Telescope RA DEC r (AU) ∆ (AU) α ( ◦ ) filter θ s (”) r (cid:48) r (cid:48) r (cid:48) r (cid:48) r (cid:48) r (cid:48) R R R R R R r R R R r (cid:48) R R R R R R R r (cid:48) R r (cid:48) Table 2 . Columns: (1) Date of observation; (2) Telescope; (3) Right Ascension at the start of observation;(4) Declination at the start of observation; (5) Heliocentric distance at start of observation; (6) Geocentricdistance at start of observation; (7) Phase Angle at start of observation; (8) Filter; (9) in-image seeing of atthe start of observation. Purdum et al. JD Mag σ Mag Observatory Table 3.
Photometric Lightcurve data for Figures 6 and 7. Columns: (1) Julian Date; (2) Normalized Magnitude; (3)Normalized Magnitude Uncertainty; (4) Location Observations were taken.
Note —Table 3 is published in its entirety in the machine-readable format. A portion is shown here for guidance regarding itsform and content.