Cometary Science Newsletter

Issue
57
Month
December 2019
Editor
Michael S. P. Kelley (msk@astro.umd.edu)

Refereed Articles

Abstracts of articles in press or recently published. Limited to 3000 characters.

Analysis of HST WFPC2 Observations of Centaur 29P/Schwassmann-Wachmann 1 while in Outburst to Place Constraints on the Nucleus's Rotation State

  • Schambeau, C. A. 1
  • Fernandez, Y. R. 1,2
  • Samarasinha, N. H. 3
  • Woodney, L. M. 4
  • Kundu, A. 5
  1. Florida Space Institute, University of Central Florida, Orlando, FL, USA.
  2. Department of Physics, University of Central Florida, Orlando, FL, USA.
  3. Planetary Science Institute, Tucson, AZ, USA.
  4. California State University - San Bernardino, San Bernardino, CA, USA.
  5. Eureka Scientific, Oakland, CA, USA.

We present analysis of Hubble Space Telescope (HST) observations of Centaur 29P/Schwassmann-Wachmann 1 (SW1) while in outburst to characterize the outburst coma and place constraints on the nucleus' spin state. The observations consist of Wide Field and Planetary Camera 2 (WFPC2) images from Cycle 5, GO-5829 (Feldman 1995) acquired on UT 1996 March 11.3 and 12.1, which serendipitously imaged the Centaur shortly after a major outburst. A multi-component coma was detected consisting of: an expanding outburst dust coma with complex morphology possessing an east-west asymmetry and north-south symmetry contained within 5″ (∼19,000 km) of the nucleus, the residual dust shell of an earlier UT 1996 February outburst, and a nearly circular underlying quiescent activity level coma detectable to ∼70″ (∼267,000 km) away from the nucleus. Photometry of the calibrated WFPC2 images resulted in a measured 5″ radius aperture equivalent R-band magnitude of 12.86 ± 0.02 and an estimated (2.79 ± 0.05)×108 kg for the lower limit of dust material emitted during the outburst. No appreciable evolution of morphologic features, indicating signatures of nucleus rotation, were detected between the two imaging epochs. The observations were modeled using a 3-D Monte Carlo coma model (Samarasinha 2000) to place constraints on the nucleus' rotation state. Modeling indicated the morphology is representative of a non-isotropic ejection of dust emitted during a single outburst event with a duration on the order of hours from a single source region corresponding to ∼1% of the surface area. A spin period with lower limit on the order of days is suggested to reproduce the coma morphology seen in the observations.

The Astronomical Journal (In press)

arXiv: 1911.00049

Comet 240P/NEAT is Stirring

  • Michael S. P. Kelley 1
  • Dennis Bodewits 2
  • Quanzhi Ye 3,4
  • Tony L. Farnham 1
  • Eric C. Bellm 5
  • Richard Dekany 6
  • Dmitry A. Duev 3
  • George Helou 4
  • Thomas Kupfer 7
  • Russ R. Laher 4
  • Frank J. Masci 4
  • Thomas A. Prince 3
  • Ben Rusholme 4
  • David L. Shupe 4
  • Maayane T. Soumagnac 8
  • Jeffry Zolkower 6
  1. Department of Astronomy, University of Maryland, College Park, MD 20742-0001, U.S.A.
  2. Physics Department, Leach Science Center, Auburn University, Auburn, AL 36832, U.S.A.
  3. Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, U.S.A.
  4. Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, U.S.A.
  5. DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195, U.S.A.
  6. Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, U.S.A.
  7. Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA
  8. Benoziyo Center for Astrophysics, Weizmann Institute of Science, Rehovot, Israel

Comets are primitive objects that formed in the protoplanetary disk, and have been largely preserved over the history of the Solar System. However, they are not pristine, and surfaces of cometary nuclei do evolve. In order to understand the extent of their primitive nature, we must define the mechanisms that affect their surfaces and comae. We examine the lightcurve of comet 240P/NEAT over three consecutive orbits, and investigate three events of significant brightening (Δm∼−2 mag). Unlike typical cometary outbursts, each of the three events are long-lived, with enhanced activity for at least 3 to 6 months. The third event, observed by the Zwicky Transient Facility, occurred in at least two stages. The anomalous behavior appears to have started after the comet was perturbed by Jupiter in 2007, reducing its perihelion distance from 2.53 to 2.12 au. We suggest that the brightening events are temporary transitions to a higher baseline activity level, brought on by the increased insolation, which has warmed previously insulated sub-surface layers. The new activity is isolated to one or two locations on the nucleus, indicating that the surface or immediate sub-surface is heterogeneous. Further study of this phenomenon may provide insight into cometary outbursts, the structure of the near-surface nucleus, and cometary nucleus mantling.

The Astrophysical Journal Letters (Published)

DOI: 10.3847/2041-8213/ab53e0 NASA ADS: 2019arXiv191102383K arXiv: 1911.02383

Diagnostics of Collisions between Electrons and Water Molecules in Near-ultraviolet and Visible Wavelengths

  • D. Bodewits 1
  • J. Országh 2
  • J. Noonan 3
  • M. Ďurian 2
  • Š. Matejčík 2
  1. Auburn U., USA
  2. Comenius U., Slovak Republic
  3. LPL, U. Arizona, USA

We studied the dissociation reactions of electron impact on water vapor for several fragment species at optical and near-ultraviolet wavelengths (200–850 nm). The resulting spectrum is dominated by the hydrogen Balmer series, by the OH (A 2Σ+ − X 2Π) band, and by the emission of ionic H2O+ (A 2A1−X 2B1) and OH+ (A 3Π−X 3Σ−) band systems. Emission cross sections and reaction channel thresholds were determined for energies between 5 and 100eV. We find that the electron impact dissociation of H2O results in an emission spectrum of the OH (A 2Σ+−X 2Π) band that is distinctly different from the emission spectra from other excitation mechanisms seen in planetary astronomy. We attribute the change to a strongly non-thermal population of rotational states seen in planetary astronomy. This difference can be utilized for remote probing of the contribution of different physical reactions in astrophysical environments.

Astrophysical Journal (Published)

DOI: 10.3847/1538-4357/ab43c9 NASA ADS: 2019arXiv190908878B arXiv: 1909.08878

First Results from TESS Observations of Comet 46P/Wirtanen

  • Tony L. Farnham 1
  • Michael S. P. Kelley 1
  • Matthew M. Knight 1
  • Lori M. Feaga 1
  1. University of Maryland, College Park, MD

We report on initial results from 20 days' worth of TESS spacecraft observations of comet 46P/Wirtanen. The long-duration, high-cadence measurements show a 2018 September 26 outburst that exhibited a two-phase, 0.5 mag brightening profile, and may be the best temporally characterized natural outburst ever recorded. Gas velocities from the outburst peaked at 800 m s−1, while dust expanded at only 10s of m s−1. Coadded images also revealed a previously unreported dust trail that extends beyond the 24 deg field of view.

Astrophysical Journal Letters (Published)

DOI: 10.3847/2041-8213/ab564d NASA ADS: 2019arXiv191108318F arXiv: 1911.08318

Initial Characterization of Interstellar Comet 2I/Borisov

  • Jewitt, D, 1
  • Luu, J. 2
  1. UCLA
  2. University of Oslo

We present initial observations of the interstellar body 2I/(2019 Q4) Borisov taken to determine its nature prior to the perihelion in 2019 December. Images from the Nordic Optical Telescope show a prominent, morphologically stable dust coma and tail. The dust cross-section within 15,000 km of the nucleus averages 130 sq.km (assuming geometric albedo 0.1) and increases by about 1 percent per day. If sustained, this rate indicates that the comet has been active for about 100 days prior to the observations. Cometary activity thus started in 2019 June, at which time C/Borisov was about 4.5 AU from the Sun, a typical distance for the onset of water ice sublimation in comets. The dust optical colors, B-V = 0.80+/-0.05, V-R = 0.47+/-0.03 and R-I = 0.49+/-0.05 are identical to those of a sample of (solar system) long-period comets. The colors are similar to those of 1I/(2017 U1) 'Oumuamua, indicating a lack of the ultrared matter that is common in the Kuiper belt, on both interstellar objects. The effective size of the dust particles is estimated as 0.1 mm, based on the length of the dust tail and the 100 day lifetime. With this size, the ejected dust mass is of order 1.3e7 kg and the current dust mass loss rate 2 kg/s. We set an upper limit to the nucleus radius using photometry at r_n < 3.8 km (again for albedo 0.1) and we use a statistical argument to show that the nucleus must be much smaller, likely a few hundred meters in radius.

Astrophysical Journal Letters 886, L29 (Published)

DOI: 10.3847/2041-8213/ab530b arXiv: 1910.02547