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Six Years of Sustained Activity from Active Asteroid (6478) Gault Colin Orion Chandler,1 Jay Kueny,1 Annika Gustafsson,1 Chadwick A

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Six Years of Sustained Activity from Active Asteroid (6478) Gault Colin Orion Chandler,1 Jay Kueny,1 Annika Gustafsson,1 Chadwick A Draft version May 3, 2019 Typeset using LATEX preprint2 style in AASTeX62 Six Years of Sustained Activity from Active Asteroid (6478) Gault Colin Orion Chandler,1 Jay Kueny,1 Annika Gustafsson,1 Chadwick A. Trujillo,1 Tyler D. Robinson,1 and David E. Trilling1 1Department of Physics & Astronomy, Northern Arizona University, PO Box 6010, Flagstaff, AZ 86011, USA ABSTRACT We present archival observations demonstrating that main belt asteroid (6478) Gault has an extensive history of comet-like activity. Outbursts have taken place during multiple epochs since 2013 and at distances extending as far as 2.68 au, nearly aphelion. (6478) Gault is a member of the predominately S-type (i.e., volatile-poor) Phocaea family; no other main belt object of this type has ever shown more than a single activity outburst. Furthermore, our data suggest this is the longest duration of activity caused by a body spinning near the rotational breakup barrier. If activity is indeed unrelated to volatiles, as appears to be the case, (6478) Gault represents a new class of object, perpetually active due to rotational spin-up. Keywords: minor planets, asteroids: individual ((6478) Gault) | comets: individual ((6478) Gault) 1. INTRODUCTION Active asteroids like (6478) Gault (Figure1, this work) are dynamically asteroidal objects but they uncharacteristically manifest cometary features such as tails or comae (Hsieh & Je- witt 2006a). With only ∼20 known to date (see Table 1 of Chandler et al. 2018), active asteroids remain poorly understood, yet they promise insight into solar system volatile dis- position and, concomitantly, the origin of water on Earth (Hsieh & Jewitt 2006b). arXiv:1904.10530v2 [astro-ph.EP] 2 May 2019 Active asteroids are often defined as objects with (1) comae, (2) semimajor axes interior to Jupiter, and (3) Tisserand parameters with re- spect to Jupiter TJ > 3; TJ describes an object's orbital relationship to Jupiter by Figure 1. (6478) Gault (dashed green circle) dis- Corresponding author: Colin Orion Chandler plays a prominent tail (indicated by white arrows) [email protected] during this 2013 September 28 apparition when (6478) Gault was halfway between perihelion and aphelion. This 90 s g-band exposure reached ∼7 mag fainter than (6478) Gault. The anti-Solar di- rection (− ; yellow) and negative heliocentric ve- locity vector (−~v; red) are shown. 2 Chandler et. al tional breakup (e.g., 311P/PanSTARRS, Jewitt s 2 aJ a (1 − e ) et al. 2013; Moreno et al. 2014), thermal frac- TJ = + 2 cos (i) (1) ture (e.g., (3200) Phaethon, discussed below), a aJ and cryovolcanism (e.g., (1) Ceres, K¨uppers where aJ is the semimajor axis of Jupiter, e et al. 2014; Witze 2015). Physical interaction, is the eccentricity, and i is the inclination; see or \rubbing binary," has been proposed as a pri- Jewitt(2012) for a thorough treatment. Main mary mechanism in the case of 311P (Hainaut belt comets are a subset of active asteroids dy- et al. 2014, cf. Jewitt et al. 2018). Secondary namically constrained to the main asteroid belt mechanisms such as electrostatic gardening, and thought to have volatile-driven activity (see physical properties like chemical makeup, and e.g., Snodgrass et al. 2017 for an in-depth dis- geometric effects (e.g., the opposition effect) cussion). It is worth pointing out that some may influence our ability to reliably detect and objects have had multiple classifications, for in- quantify outbursts. stance (3552) Don Quixote has an asteroid des- One crucial diagnostic indicator of the under- ignation due to its low activity and has been lying activity mechanism is whether or not ac- called a near-Earth asteroid (Mommert et al. tivity recurs. If activity is observed on only 2014) but has a TJ of 2.3 which indicates it is one occasion (i.e., a single apparition), then more properly a Jupiter family comet. the object may have experienced a recent im- Discovering these objects has proven obser- pact event. Expelled material and/or exposed vationally challenging. The first active aster- volatiles sublimating may both cause comae or oid, (4015) Wilson-Harrington, was discovered tails to appear. Activity would then cease once in 1949 (Harris 1950). In the mid-1980s a con- ejecta dissipated or the volatile supply is ex- nection between bow-shock magnetic field dis- hausted, reburied, or refrozen. turbances detected by the Pioneer spacecraft Recurrent activity is typically associated with suggested (2201) Oljato was leaving behind a volatile sublimation. For example, Geminid distant comet-like gas trail (Kerr 1985), even if Meteor Shower parent body (3200) Phaethon not detected at the object itself (Russell et al. is thought to undergo thermal fracture during 1984). Despite many efforts (see e.g., Cham- the rapid temperature changes accompanying berlin et al. 1996) it was not until the 1996 its perihelion passages (Li & Jewitt 2013) where discovery of activity in (7968) Elst-Pizarro that it experiences temperatures > 800 K (Ohtsuka another active asteroid was visually identified et al. 2009). Fracture events may directly ex- (Elst et al. 1996). Though initially impact ap- pel material in addition to exposing volatiles for peared a possible cause (e.g., Toth 2000), when sublimation. activity recurred (Hsieh et al. 2010) it was more Thermally induced activity is thought to in- indicative of volatile sublimation. crease with decreasing heliocentric distance; A significant complication hindering our un- that is, the closer a body is to the Sun, the derstanding of active asteroids arises when as- more likely an outburst is to occur. Active as- sessing underlying activity mechanisms: causes teroids are more likely to exhibit activity dur- are neither few nor mutually exclusive (see Je- ing perihelion passage (see Table 1 of Chan- witt 2012 for a comprehensive overview). Re- dler et al. 2018). Notable exceptions where sponsible primary processes include volatile activity was discovered at distances far from sublimation (e.g., 133P/Elst-Pizarro, Hsieh perihelion include 311P/PanSTARRS (Jewitt et al. 2010), impact events (e.g., (596) Scheila, et al. 2013) and (493) Griseldis (Tholen et al. Bodewits et al. 2011; Moreno et al. 2011), rota- Six Years of Sustained Activity from Active Asteroid (6478) Gault 3 2015). Activity in \traditional" comets has We searched our own in-house database of been reported at distances that are substan- archival astronomical data (e.g., observation tially farther than the main asteroid belt, for date, coordinates) in order to locate images instance Comet C/2010 U3 (Boattini) at 27 au that are likely to show (6478) Gault. Our (Hui et al. 2019). Of the ∼20 active asteroids database includes the entire NOAO DECam known to date, 16 are carbonaceous (i.e., C- public archive data tables along with corre- type) but only four are believed to be com- sponding data from myriad sources (e.g., NASA posed of silicate-rich non-primitive material JPL Horizons Giorgini et al. 1996; see also the (i.e., DeMeo et al. 2009 S-type taxonomy): Acknowledgements). (2201) Oljato (Apollo-orbit), 233P/La Sagra 2.1. Locating Candidate Images (Encke-orbit), 311P/PanSTARRS (inner main belt), and 354P/LINEAR (outer main belt). We began our search for (6478) Gault by mak- (6478) Gault activity was first reported in ing use of a fast grid query in R.A. and decl. 2019 January (Smith et al. 2019). Ensuing space. We then passed these results through follow-up observations (Maury et al. 2019) con- a more accurate circular filter prescribed for firmed activity with subsequent reports (Lee the DECam image sensor arrangement. Lastly, 2019; Ye et al. 2019a) providing evidence of we computed image sensor chip boundaries pre- ongoing activity. Ye et al.(2019b) reported cisely to ensure that the object fell on a sensor that multiple outbursts actually began in 2018 rather than, for example, gaps between camera December. Analysis of dust emanating from chips. This progressively more precise query ap- (6478) Gault via Monte Carlo tail brightness proach cut down image search time by orders of simulations indicate the current apparition, magnitude. comprised of two outbursts, could have begun 2.2. Observability Assessment as early as 2018 November 5 (Moreno et al. 2019). Simultaneous to our own work, Jewitt We created a reverse exposure time calcula- et al.(2019) reported three tails with indepen- tor to estimate how faintly (i.e., the magnitude dent onsets, the earliest being 2018 October limit) candidate images probed. After applying 28. color coefficient corrections (see Willmer 2018 We set out to determine if any data in our for procedure details) we transformed the color- local repository of National Optical Astron- corrected magnitudes to the absolute bolomet- omy Observatory (NOAO) Dark Energry Cam- ric system used by the DECam exposure time 1 era (DECam) images showed signs of activity. calculator . These steps enabled us to com- The ∼500 megapixel DECam instrument on the pute differences between apparent magnitude Blanco 4 m telescope situated on Cerro Tololo, and the specific magnitude limit of the DECam Chile, probes faintly (∼24 mag) and, as we exposure so that we could produce a list of im- demonstrated in Chandler et al.(2018), it is ages where (6478) Gault could be detected. well-suited to detect active asteroids. We pro- 2.3. Thumbnail Extraction duced novel tools taking into account (1) orbital We downloaded the image files containing properties of (6478) Gault (summarized in Ap- (6478) Gault from the NOAO archive and, fol- pendixA) and (2) observational properties (e.g., lowing the procedures of Chandler et al.(2018), apparent magnitude, filter selection, exposure we extracted flexible image transport system time) to find ideally suited candidate images. 2. METHODS 1 http://www.ctio.noao.edu/noao/node/5826 4 Chandler et. al (FITS) thumbnails of (6478) Gault.
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