October 12, 2018: 50th Division For Planetary Sciences Meeting Practice Talks

Practice Talks for the upcoming DPS meeting. Presentations given by:
Ariel Graykowski: Fragmenting Comet 73P
Man-To Hui: Ultra-Distant Activity in Comet C/2017 K2 (PANSTARRS)
Dave Milewski: Continuous Monitoring of Active Asteroid P/2016 G1 (Pan-STARRS)
Ariel Graykowski
73P/Schwassmann-Wachmann 3 is a Jupiter-family comet that has been observed to fragment on several occasions since 2005. Fragment C (73P-C) is believed to be the primary component of the nucleus. Knowing the nucleus rotation period is important because it can provide invaluable clues about the mechanism responsible for the breakup of this comet. Unfortunately, studies of 73P-C using a variety of techniques have reported rotational periods that range over an order of magnitude, from about 2.8 to 27.2 hours. The lower end of the reported range is compatible with rotational breakup of a low strength nucleus whereas the higher end would rule that possibility out completely. We have undertaken a systematic analysis of unpublished archival Hubble Space Telescope data from 2006 April in order to determine the rotation period and to assess other aspects of 73P-C. We find strong, cyclic photometric variations of about 0.4 magnitudes in the central light from this object. Similar variations with a smaller range are apparent in the surrounding dust coma, compatible with rotational modulation of the mass loss rate. I will discuss our measurements and inferences.
Man-To Hui
C/2017 K2 (PANSTARRS; hereafter “K2”) is an inbound Oort cloud comet exhibiting activity at least from 23.7 AU, a record heliocentric distance. We are studying the development of the activity using the Hubble Space Telescope and data scoured from the electronic archives. Our HST observations since 2017 June reveal a circularly symmetric dust coma ~10^5 km in radius, with a total effective cross-section ~10^5 {\textrm{km}^2}. The coma logarithmic surface brightness gradient is -1.01±0.01, consistent with the value expected for steady-state mass loss. The absence of a radiation-pressure caused tail suggests that the average ejected dust size is very large. Our Monte Carlo simulations indicate a mean dust diameter of ~1 mm, and an ejection speed of only a few m/s. We estimate the nucleus to be several kilometers in radius. Activity in K2, which we find is losing mass at ~10^2 kg/s, cannot be driven by the sublimation or crystallization of water ice. Instead, the sublimation of supervolatiles including CO and CO2 is suspected. Our numerical integrations show that the previous perihelion occurred >1 Myr ago, and therefore no heat from the prior orbit can be retained. Continuing observations will probe the development of activity as K2 approaches the Sun.
Dave Milewski
We present a study of active asteroid P/2016 G1 (Pan-STARRS) (‘G1’) conducted between 2016 April and August. Our ground-based data were augmented by archival Hubble Space Telescope data as well as follow-up monitoring after G1 passed perihelion on UT 26 January 2017. The active asteroids are a subset of main belt asteroids that share the dynamical properties of asteroids but also the physical properties of comets such as the appearance of dust tails. By the use of the Jeanne Rich Centurion 28 0.7-m Telescope, the Keck-I 10-m telescope, and HST, data were obtained on 11 different epochs. We find that 1) G1’s apparent R-band magnitude faded from ∼17.8 to ∼19.6 over a period of three months and steeply dropped between UT 2016 June and UT 2016 August, 2) the absolute magnitude (assuming a phase function β = 0.02 magnitudes per degree) varied from 14.7 ± 0.09 on UT 17 April 2016 to 15.9 ± 0.02 in our last useful observation on UT 2016 August 03, 3) our photometry suggests that about 107 kg dust was ejected, and 4) the likely mechanism of activation remains impact, as suggested earlier by Moreno et al.

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