Draft version August 3, 2020 Typeset using LATEX twocolumn style in AASTeX63 Color changes for the Surface of 6478 Gault (??) 1 2 2 3 4 5 1 Remington Cantelas, Karen J. Meech, Jan T. Kleyna, Erica Bufanda, Alan Fitzsimmons, James Bauer, 2 2 2 6 2 2 Larry Denneau, Robert Weryk, Jacqueline V. Keane, Olivier R. Hainaut, and Richard J. Wainscoat 1 3 University of Central Florida, 4111 Libra Drive, Orlando, FL 32816, USA 2 4 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA 3 5 Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822 USA 4 6 Astrophysics Research Centre, Queen's University Belfast, Belfast BT7 1NN, UK 5 7 University of Maryland, Dept. of Astronomy, College Park, MD 20742-2421 USA 6 8 European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei M¨unchen,Germany 9 Submitted to ApJL 10 ABSTRACT 11 (6478) Gault is a main belt asteroid in the Phocaea Family which was discovered to have activity in 12 January 2019, and precovery images reveal it has been consistently active since at least September 2013. 13 Gault's activity is believed to be caused by it being a fast rotator near the asteroid break-up limit. We 14 have collected images and photometry from several telescopes dating back several apparitions. Using 15 this data we attempt to produce a reliable rotational light curve and confirm Gault's rotation period. 16 We also investigate possible color variations on the surface of the asteroid by measuring it's spectral 17 reflectivity over the course of 7 months between January and August 2019. These color variations would 18 imply that Gault's activity is either revealing new fresh material unaffected by long-term radiation 19 from the Sun, or Gault's surface is composed of a mineralogy we would not typically expect for an 20 asteroid of it's class. Confirming these color variations could have significant implications in the field 21 of space weathering and would also be notable since color variations on an asteroids surface have not 22 been observed from the ground before. 23 Keywords: minor planets, asteroids: individual ((6478) Gault) | planets and satellites: dynamical 24 evolution and stability 25 1. INTRODUCTION 43 impact events, rotational breakup, thermal fracture, and 44 \rubbing binaries" (Jewitt 2012). 26 On January 5th 2019, the ATLAS sky survey de- 45 Determining Gault's asteroid type will give us insight 27 tected activity around the main belt asteroid (6478) 46 into what the object might be made of and may help 28 Gault. Further investigation showed the development of 47 constrain likely sources of activity. For instance, most 29 three separate tails: the first appearing on October 28 48 cometary activity is associated with volatile sublimation 30 2018, the second on December 31 2018, and the third on 49 caused by ices in the object sublimating due to heat- 31 February 10 2019 (Jewitt et al. 2019). Looking back at 50 ing from the Sun. This type of activity is more likely 32 existing NOAO DECam images, Chandler et al.(2019) 51 to occur on primitive C-type asteroids which are gen- 33 found that activity in the asteroid had been consistent 52 erally thought to have formed farther out in the aster- 34 since at least 2013. These discoveries make Gault a rare 53 oid belt where temperatures are cooler and volatiles are 35 new member of a small group of active asteroids. 54 more likely to condense. Furthermore, objects under- 36 Active asteroids are defined as objects that exhibit 55 going volatile sublimation show more activity near per- 37 comet-like behavior, have semi-major axes within the 56 ihelion and less during aphelion. In the case of Gault, 38 orbit of Jupiter, and a Tisserand parameter of Tj > 3 57 the brightness due to activity varies along it's orbit, but 39 (Jewitt et al. 2015). There are only ∼ 20 known active 58 this brightness has no correlation to Gault's distance 40 asteroids, 16 of which are C-type and 4 are S-type (Je- 59 from the Sun (Chandler et al. 2019). Optical reflectance 41 witt 2012). Activity on these asteroids can be caused by 60 spectra showed no gas in Gault's tail (Jewitt et al. 2019). 42 a variety of phenomena, including: volatile sublimation, 2 61 This evidence argues against volatile sublimation as the Table 1. Observations 62 main cause for Gault's activity. 00 63 Gault is a member of the Phocaea family (Nesvorny Telescope Detector Gain RN /pix 64 2015), made up mostly of S-type objects and the ATLAS STA-1600 2.0 11.0 1.86 65 low albedo, presumed C-type, Tamara sub-family (No- Gemini GMOS 2.27 3.32 0.161 66 vakovi´cet al. 2017). Color observations from early 2019 CFHT Megacam 1.634 3.00 0.187 67 suggested that Gault was most likely a C-type asteroid PanSTARRS GPC1 1.256 7.462 0.260 68 (Kleyna et al. 2019; Hui et al. 2019; Lee 2019; Jewitt VLT FORS2 0.8 2.7 0.126 69 et al. 2019). Later, visible and near-Infrared spectra 70 taken in March and April of 2019, after activity had de- 112 (ATLAS), the All Sky Automated Survey for Super- 71 creased, suggested instead that Gault was very clearly 113 Novae (ASAS-SN) and the Very Large Telescope (VLT). 72 an S-type, observing the two distinct 1µm and 2µm ab- 114 All images, with the exception of VLT and Gemini im- 73 sorptions bands associated with silicates (Marsset et al. 115 ages, were reduced using our pipeline. To photometri- 74 2019). Curiously, Marsset et al.(2019) also found that 116 cally calibrate the data we calculated a photometric zero 75 the color had changed significantly, from a very blue- 117 point for each image using the Pan-STARRS, SDSS and 76 sloped Q-type spectra on March 31 2019 to a typical 118 Gaia2 catalogs and published color corrections to trans- 77 red S-type spectra on April 8 2019. Carbognani & Buz- 119 late photometric bands (Magnier et al. 2016; Chambers 78 zoni(2020) found a similar blue color shift on April 15, 120 et al. 2016). The information from the image head- 79 2019 using optical photometry. 121 ers was used to download orbital elements from the 80 One suggested mechanism for activity on Gault is 122 Minor Planet Center and the computed object loca- 81 rotational breakup caused by the Yarkovsky{OKeefe{ 123 tion was used to determine which object in the frame 82 Radzievskii{Paddack (YORP) effect. Asteroids absorb 124 corresponded to the target. Terapix tools (SExtractor 83 sunlight and re-emit it as thermal radiation, and over 125 (Bertin & Arnouts 1996)) were used to produce multi- 84 long periods time the momentum from this radiation can 126 aperture and automatic aperture target photometry for 85 gradually speed up an asteroid's rotation until the ap- 127 several of the data sets. A full description of the pipeline 86 parent surface gravity is zero (Bottke et al. 2006). This 128 is given in Meech et al.(2017). Gemini data was reduced 87 can trigger disruption or landslide events that would re- 129 using the new Gemini DRAGONS reduction software. 88 lease dust at a near zero velocity which would get swept 130 For all telescopes except VLT we photometrically cal- 89 away by solar radiation pressure, hence producing a tail. 131 ibrate our images by calculating a zero point for each 90 Several dust dynamical models confirm this, finding that 132 image using the Pan-STARRS database and published 91 the dust in Gault's tail has a low ejection velocity (Je- 133 color corrections to translate photometric bands. The 92 witt et al. 2019; Hui et al. 2019; Kleyna et al. 2019; 134 photometry is shown in Figure7. 93 Moreno et al. 2019; Ye et al. 2019). Kleyna et al.(2019) 135 Photometry was done using the IRAF software to de- 94 has proposed a ∼ 2 hour rotation period, which is near 136 termine the best aperture size for each set of data. For 95 the limit of a body with some internal cohesion (Holsap- 137 faint objects this is done by finding the curve of growth 96 ple 2007; Chang et al. 2019). Other studies have been 138 for an unsaturated star in the image and determining 97 unable to confirm this rotation period, but this is mostly 139 the aperture at which most light is collected with the 98 due to the effect of dust masking around Gault making 140 least amount of sky. In addition, smaller aperture sizes 99 it difficult to detect a brightness variation from the ro- 141 can decrease the amount of contribution coming from 100 tating nucleus (Kleyna et al. 2019; Jewitt et al. 2019; Ye 142 the dust cloud surrounding an object and increase the 101 et al. 2019; Sanchez et al. 2019; Moreno et al. 2019). 143 contribution from the nucleus. This technique could al- 102 In this paper, we will be looking at data from before 144 leviate the effects of dusk masking on our light curve 103 and after Gault's 2019 period of activity to try to deter- 145 and reveal periodic rotational variabilities which were 104 mine it's rotation period. We will also be looking deeper 146 not found in previous studies. 105 into Gault's colors to find out what mechanisms might 106 be causing such drastic color variations. 147 2.1.