Cometary Science Newsletter

Issue
29
Month
August 2017
Editor
Michael S. P. Kelley (msk@astro.umd.edu)

Refereed Articles

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

Rotation of Cometary Nuclei: New Lightcurves and an Update of the Ensemble Properties of Jupiter-Family Comets

  • Kokotanekova, R. 1,2
  • Snodgrass, C. 2
  • Lacerda, P. 3
  • Green, S. F. 2
  • Lowry, S. C. 4
  • Fernandez, Y. R. 5
  • Tubiana, C. 1
  • Fitzsimmons, A. 3
  • Hsieh, H. H. 6,7
  1. Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077, Goettingen, Germany.
  2. Planetary and Space Sciences, School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK.
  3. Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK.
  4. Centre for Astrophysics and Planetary Science, School of Physical Sciences (SEPnet), The University of Kent, Canterbury, CT2 7NH, UK.
  5. Dept. of Physics, Univ. of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32816-2385, USA.
  6. Planetary Science Institute, 1700 E. Ft. Lowell Road, Suite 106, Tucson, AZ 85719, USA.
  7. Institute of Astronomy and Astrophysics, Academia Sinica, P.O. Box 23-141, Taipei 10617, Taiwan

We report new lightcurves and phase functions for nine Jupiter-family comets (JFCs). They were observed in the period 2004-2015 with various ground telescopes as part of the Survey of Ensemble Physical Properties of Cometary Nuclei (SEPPCoN) as well as during devoted observing campaigns. We add to this a review of the properties of 35 JFCs with previously published rotation properties.

The photometric time-series were obtained in Bessel R, Harris R and SDSS r' filters and were absolutely calibrated using stars from the Pan-STARRS survey. This specially-developed method allowed us to combine data sets taken at different epochs and instruments with absolute-calibration uncertainty down to 0.02 mag. We used the resulting time series to improve the rotation periods for comets 14P/Wolf, 47P/Ashbrook-Jackson, 94P/Russell, and 110P/Hartley 3 and to determine the rotation rates of comets 93P/Lovas and 162P/Siding Spring for the first time. In addition to this, we determined the phase functions for seven of the examined comets and derived geometric albedos for eight of them.

We confirm the known cut-off in bulk densities at 0.6 g cm-3 if JFCs are strengthless. Using the model of Davidsson (2001) for prolate ellipsoids with typical density and elongations, we conclude that none of the known JFCs require tensile strength larger than 10-25 Pa to remain stable against rotational instabilities. We find evidence for an increasing linear phase function coefficient with increasing geometric albedo. The median linear phase function coefficient for JFCs is 0.046 mag/deg and the median geometric albedo is 4.2 per cent.

Monthly Notices of the Royal Astronomical Society (In press)

NASA ADS:  2017arXiv170702133K arXiv: 1707.02133

Deposition of steeply infalling debris around white dwarf stars

  • John C. Brown 1
  • Dimitri Veras 2
  • Boris T. Gänsicke 2
  1. School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
  2. Department of Physics, University of Warwick, Coventry CV4 7AL, UK

High-metallicity pollution is common in white dwarf (WD) stars hosting remnant planetary systems. However, they rarely have detectable debris accretion discs, possibly because much of the influx is fast steeply infalling debris in star-grazing orbits, producing a more tenuous signature than a slowly accreting disc. Processes governing such deposition between the Roche radius and photosphere have so far received little attention and we model them here analytically by extending recent work on sun-grazing comets to WD systems. We find that the evolution of cm-to-km size (a0) infallers most strongly depends on two combinations of parameters, which effectively measure sublimation rate and binding strength. We then provide an algorithm to determine the fate of infallers for any WD, and apply the algorithm to four limiting combinations of hot versus cool (young/old) WDs with snowy (weak, volatile) versus rocky (strong, refractory) infallers. We find: (i) Total sublimation above the photosphere befalls all small infallers across the entire WD temperature (TWD) range, the threshold size rising with TWD and 100× larger for rock than snow. (ii) All very large objects fragment tidally regardless of TWD: for rock, a0 ≽ 105 cm; for snow, a0 ≽ 103 - 3 × 104 cm across all WD cooling ages. (iii) A considerable range of a0 avoids fragmentation and total sublimation, yielding impacts or grazes with cold WDs. This range rapidly narrows with increasing TWD, especially for snowy bodies. Finally, we briefly discuss how the various forms of deposited debris may finally reach the photosphere surface itself.

Monthly Notices of the Royal Astronomical Society (Published)

DOI: 10.1093/mnras/stx428 NASA ADS: 2017MNRAS.468.1575B arXiv: 1702.05109

Debiasing the NEOWISE Cryogenic Mission Comet Populations

  • James M. Bauer 1,2,5
  • Tommy Grav 3
  • Yanga R. Fernández 4
  • A. K. Mainzer 1
  • Emily A. Kramer 1
  • and 14 co-authorsNone
  1. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 183-401, Pasadena, CA 91109, USA; gerbsb@astro.umd.edu
  2. IPAC, California Institute of Technology, Pasadena, CA 91125, USA
  3. Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA
  4. Department of Physics, University of Central Florida, 4000 Central Florida Boulevard, P.S. Building, Orlando, FL 32816-2385, USA
  5. Department of Astronomy University of Maryland Atlantic Building 224, Room 1245 College Park, MD 20742.

We use NEOWISE data from the four-band and three-band cryogenic phases of the Wide-field Infrared Survey Explorer mission to constrain size distributions of the comet populations and debias measurements of the short- and long-period comet (LPC) populations. We find that the fit to the debiased LPC population yields a cumulative size!frequency distribution (SFD) power-law slope (beta) of 1.0±0.1, while the debiased Jupiter-family comet (JFC) SFD has a steeper slope with beta=2.3±0.2. The JFCs in our debiased sample yielded a mean nucleus size of 1.3 km in diameter, while the LPCs’ mean size is roughly twice as large, 2.1 km, yielding mean size ratios (<LPC>/<JFC>) that differ by a factor of 1.6. Over the course of the 8 months of the survey, our results indicate that the number of LPCs passing within 1.5 au are a factor of several higher than previous estimates, while JFCs are within the previous range of estimates of a few thousand down to sizes near 1.3 km in diameter. Finally, we also observe evidence for structure in the orbital distribution of LPCs, with an overdensity of comets clustered near 110 degrees inclination and perihelion near 2.9 au that is not attributable to observational bias.

The Astronomical Journal (Published)

DOI: 10.3847/1538-3881/aa72df NASA ADS: 2017AJ....154...53B

Origin and Evolution of Short-Period Comets

  • David Nesvorny 1
  • David Vokrouhlicky 2
  • Luke Dones 1
  • Harold F. Levison 1
  • Nathan Kaib 3
  • Alessandro Morbidelli 4
  1. Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA
  2. Institute of Astronomy, Charles University, V Holesovickach 2, CZ--18000 Prague 8, Czech Republic
  3. HL Dodge Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019, USA
  4. Departement Cassiopee, University of Nice, CNRS, Observatoire de la Cote d'Azur, Nice, 06304, France

Comets are icy objects that orbitally evolve from the trans-Neptunian region (the Kuiper belt and beyond) into the inner Solar System, where they are heated by solar radiation and become active due to sublimation of water ice. Here we perform end-to-end simulations in which cometary reservoirs are formed in the early Solar System and evolved over 4.5 Gyr. The gravitational effects of Planet 9 (P9), hypothesized to circle the Sun on a wide orbit, are included in some of our simulations. Different models are considered for comets to be active, including a simple assumption that comets remain active for Np(q) perihelion passages with perihelion distance q<2.5 au. The orbital distribution and number of active comets produced in our model is compared to observations. The orbital distribution of ecliptic comets (ECs) is well reproduced in models with Np(2.5)=500 and without P9. With P9, the inclination distribution of model ECs is wider than the observed one. We find that the known Halley-type comets (HTCs) have a nearly isotropic inclination distribution (with only a slight preference for prograde orbits). In our model, the HTCs appear to be an extension of the population of returning Oort-cloud comets (OCCs) to shorter orbital periods. The inclination distribution of model HTCs becomes broader with increasing Np, but the existing observational data are not good enough to constrain Np from orbital fits. Np(2.5)>1000 is required to obtain a steady-state population of large active HTCs that is consistent with observations. To fit the ratio of the returning-to-new OCCs, by contrast, our model implies that Np(2.5)<10, possibly because the detected long-period comets are smaller and much easier to disrupt than observed HTCs.

The Astrophysical Journal (In press)

NASA ADS: 2017arXiv170607447N arXiv: 1706.07447