University of Dundee Six Outbursts of Comet 46P/Wirtanen Kelley, Michael SP
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University of Dundee Six Outbursts of Comet 46P/Wirtanen Kelley, Michael S. P.; Farnham, Tony L.; Li, Jian-Yang; Bodewits, Dennis; Snodgrass, Colin; Allen, Johannes Publication date: 2021 Licence: CC BY Document Version Other version Link to publication in Discovery Research Portal Citation for published version (APA): Kelley, M. S. P., Farnham, T. L., Li, J-Y., Bodewits, D., Snodgrass, C., Allen, J., Bellm, E. C., Coughlin, M. W., Drake, A. J., Duev, D. A., Kupfer, T., Masci, F. J., Reiley, D., Walters, R., Dominik, M., Jørgensen, U. G., Bach- Møller, N., Bozza, V., Burgdorf, M. J., ... Unda-Sanzana, E. (2021, May). Six Outbursts of Comet 46P/Wirtanen. Cornell University. General rights Copyright and moral rights for the publications made accessible in Discovery Research Portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from Discovery Research Portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain. • You may freely distribute the URL identifying the publication in the public portal. Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Download date: 24. Sep. 2021 Draft version May 13, 2021 Typeset using LATEX modern style in AASTeX63 Six Outbursts of Comet 46P/Wirtanen Michael S. P. Kelley,1 Tony L. Farnham,1 Jian-Yang Li (NPl),2 Dennis Bodewits,3 Colin Snodgrass,4 Johannes Allen,3 | Eric C. Bellm,5 Michael W. Coughlin,6 Andrew J. Drake,7 Dmitry A. Duev,8 Matthew J. Graham,8 Thomas Kupfer,9 Frank J. Masci,10 Dan Reiley,11 Richard Walters,11 Zwicky Transient Facility Collaboration M. Dominik,12 U. G. Jørgensen,13 A. Andrews,14 N. Bach-Møller,13 V. Bozza,15, 16 M. J. Burgdorf,17 J. Campbell-White,18 S. Dib,13, 19 Y. I. Fujii,20, 21 T. C. Hinse,22, 23 M. Hundertmark,24 E. Khalouei,25 P. Longa-Pena,~ 26 M. Rabus,27 S. Rahvar,25 S. Sajadian,28 J. Skottfelt,14 J. Southworth,29 J. Tregloan-Reed,30 E. Unda-Sanzana,26 MiNDSTEp Collaboration 1Department of Astronomy, University of Maryland, College Park, MD 20742-0001, USA 2Planetary Science Institute, 1700 E. Ft. Lowell Rd., Tucson, AZ 85719, USA 3Physics Department, Leach Science Center, Auburn University, Auburn, AL 36832, USA 4Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK 5DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, USA 6School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA 7Department of Astronomy, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA 8Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA 9Texas Tech University, Department of Physics & Astronomy, Box 41051, 79409, Lubbock, TX, USA 10IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA 11Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA arXiv:2105.05826v1 [astro-ph.EP] 12 May 2021 12Centre for Exoplanet Science, SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK 13Centre for ExoLife Sciences (CELS), Niels Bohr Institute, Øster Voldgade 5, 1350 Copenhagen, Denmark 14Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK 15Dipartimento di Fisica "E.R. Caianiello", Universit`adi Salerno, Via Giovanni Paolo II 132, 84084, Fisciano, Italy 16Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Napoli, Italy 17Universit¨atHamburg, Faculty of Mathematics, Informatics and Natural Sciences, Department of Earth Sciences, Meteorological Institute, Bundesstraße 55, 20146 Hamburg, Germany [email protected] 2 Kelley et al. 18SUPA, School of Science and Engineering, University of Dundee, Nethergate, Dundee DD1 4HN, UK 19Max Planck Institute for Astronomy, K¨onigstuhl 17, D-69117 Heidelberg, Germany 20Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8602, Japan 21Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Nihonmatsu, Sakyo, Kyoto 606-8501, Japan 22Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland 23Chungnam National University, Department of Astronomy & Space Science, 34134 Daejeon, South Korea 24Astronomisches Rechen-Institut, Zentrum f¨urAstronomie der Universit¨atHeidelberg (ZAH), 69120 Heidelberg, Germany 25Department of Physics, Sharif University of Technology, PO Box 11155-9161 Tehran, Iran 26Centro de Astronom´ıa(CITEVA), Universidad de Antofagasta, Av. Angamos 601, Antofagasta, Chile 27Departamento de Matem´atica y F´ısica Aplicadas, Universidad Cat´olica de la Sant´ısima Concepci´on,Alonso de Rivera 2850, Concepci´on,Chile 28Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran 29Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK 30Instituto de Investigaci´onen Astronomia y Ciencias Planetarias, Universidad de Atacama, Copiap´o,Atacama, Chile (Received September 30, 2020; Revised April 19, 2021; Accepted May 03, 2021) Submitted to The Planetary Science Journal ABSTRACT Cometary activity is a manifestation of sublimation-driven processes at the surface of nuclei. However, cometary outbursts may arise from other processes that are not necessarily driven by volatiles. In order to fully understand nuclear surfaces and their evolution, we must identify the causes of cometary outbursts. In that context, we present a study of mini-outbursts of comet 46P/Wirtanen. Six events are found in our long-term lightcurve of the comet around its perihelion passage in 2018. The apparent strengths range from −0:2 to −1:6 mag in a 500 radius aperture, and correspond to dust masses between ∼ 104 to 106 kg, but with large uncertainties due to the unknown grain size distributions. However, the nominal mass estimates are the same order of magnitude as the mini-outbursts at comet 9P/Tempel 1 and 67P/Churyumov- Gerasimenko, events which were notably lacking at comet 103P/Hartley 2. We com- pare the frequency of outbursts at the four comets, and suggest that the surface of 46P has large-scale (∼10{100 m) roughness that is intermediate to that of 67P and 103P, if not similar to the latter. The strength of the outbursts appear to be corre- lated with time since the last event, but a physical interpretation with respect to solar insolation is lacking. We also examine Hubble Space Telescope images taken about 2 days following a near-perihelion outburst. No evidence for macroscopic ejecta was found in the image, with a limiting radius of about 2-m. 1. INTRODUCTION 3 Comet 46P/Wirtanen is a small Jupiter-family comet that has been considered as a potential spacecraft target. The effective radius is 0.6 km (Lamy et al. 1998; Boehnhardt et al. 2002), making it one of the smallest periodic comets (Snodgrass et al. 2011). The comet made an historic flyby of Earth in 2018, passing just 0.0775 au away (1.16×107 km) on 2018 December 16 (JPL Horizons orbital solution K181/21). The geometry with respect to the Earth and Sun was exceptionally favorable, with long observing opportunities and a total apparent magnitude peaking near V ∼ 5 mag (IAU Minor Planet Center Database). In many respects, comet Wirtanen is considered a near-twin of comet 103P/Hartley 2. They have similar orbits, dust and gas production rates, and nuclear radii (A'Hearn et al. 1995, 2011). As a consequence, both comets are considered to be hyperactive, i.e., their water production rates suggest a sublimating surface area comparable to the total nuclear surface area, whereas most comets have a ratio . 10% (A'Hearn et al. 1995). Comet Hartley 2 was a flyby target of the Deep Impact spacecraft (A'Hearn et al. 2011) and the subject of a large observational campaign in 2010 (Meech et al. 2011). Thus, the 2018 perihelion passage of comet Wirtanen presented an opportunity to apply the knowledge gained from the studies of comet Hartley 2 to comet Wirtanen and the broader comet population. One important difference between Wirtanen and Hartley 2 is the lack of cometary outbursts in the latter (A'Hearn et al. 2011). Cometary outbursts are brief increases in mass loss (Hughes 1990), instigated by mechanical or thermophysical processes, such as cliff collapse (Pajola et al. 2017), avalanches (Steckloff & Melosh 2016), nuclear fragmentation (Boehnhardt 2004), or structural failure and release of pressure from a sub-surface gas reservoir (Agarwal et al. 2017), charged by, e.g., water ice phase changes (Patashnick 1974; Prialnik & Bar-Nun 1990) or gas dissolution from a liquid (Miles 2016). Outbursts of many comets have been observed, e.g., comets Kohoutek 1973f, Bowell 1980b, 9P/Tempel 1, and 67P/Churyumov-Gerasimenko (A'Hearn & Cowan 1975; A'Hearn et al. 1984, 2005, 2016), but none have been confirmed for comet Hartley 2. This result is in spite of the 2010 observational campaign, and near-continuous photometry from the Deep Impact spacecraft. In contrast, clear outbursts of comet Wirtanen were observed in 1991, 2002, 2008, and 2018 (Yoshida 2013; Kidger 2004, 2008; Kronk et al. 2017; Combi et al. 2020; Farnham et al. 2019). Dense, long-term photometric and spectroscopic coverage of comets is needed to advance our understanding of cometary activity (A'Hearn 2017). Present-day wide- field time-domain surveys, such as the Zwicky Transient Facility (ZTF; Bellm et al. 2019a; Graham et al. 2019) and the Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018a), can partially address this challenge with broad-band photometric imaging at a near-daily cadence.