Distant EKOs, Issue #95  (October-ish 2014)


News & Announcements
Abstracts of 5 Accepted Papers
Job Announcements
Newsletter Information


After many years of ground and space-based KBO searches for a target that the New Horizons spacecraft could reach if it gets an extended mission, NASA, STScI, and the New Horizons team announced the discovery - using the Hubble Space Telescope - of some ``potentially targetable'' candidates for a flyby after next year's encounter with Pluto.


There were 6 new TNO discoveries announced since the previous issue of Distant EKOs:

2013 RB98, 2014 QL441, 2014 QM441, 2014 QN441, 2014 UH192, 2014 UM33

and 8 new Centaur/SDO discoveries:

2014 ON6, 2014 QF433 2014 RE12, 2014 SR303, 2014 SS303, 2014 SW223, 2014 TZ33, 2014 UT114

Reclassified objects:

2014 QF433 (SDO $\rightarrow$ TNO)

Objects recently assigned numbers:

2001 QJ298 = (408832)
2005 VJ119 = (413666)

Deleted/Re-identified objects:

2006 HH123

Current number of TNOs: 1284 (including Pluto)
Current number of Centaurs/SDOs: 407
Current number of Neptune Trojans: 9

Out of a total of 1700 objects:
   656 have measurements from only one opposition
     633 of those have had no measurements for more than a year
       326 of those have arcs shorter than 10 days
(for more details, see: http://www.boulder.swri.edu/ekonews/objects/recov_stats.jpg)


Formation of Pebble-Pile Planetesimals
Karl Wahlberg Jansson1 and Anders Johansen1

1 Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, SE-221 00 Lund, Sweden

Asteroids and Kuiper belt objects are remnant planetesimals from the epoch of planet formation. The first stage of planet formation is the accumulation of dust and ice grains into mm-cm-sized pebbles. These pebbles can clump together through the streaming instability and form gravitationally bound pebble `clouds'. Pebbles inside such a cloud will undergo mutual collisions, dissipating energy into heat. As the cloud loses energy, it gradually contracts towards solid density. We model this process and investigate two important properties of the collapse: (i) the timescale of the collapse and (ii) the temporal evolution of the pebble size distribution. Our numerical model of the pebble cloud is zero-dimensional and treats collisions with a statistical method. We find that planetesimals with radii larger than $\sim$100 km collapse on the free-fall timescale of about 25 years. Lower-mass clouds have longer pebble collision timescales and collapse much more slowly, with collapse times of a few hundred years for 10-km-scale planetesimals and a few thousand years for 1-km-scale planetesimals. The mass of the pebble cloud also determines the interior structure of the resulting planetesimal. The pebble collision speeds in low-mass clouds are below the threshold for fragmentation, forming pebble-pile planetesimals consisting of the primordial pebbles from the protoplanetary disk. Planetesimals above 100 km in radius, on the other hand, consist of mixtures of dust (pebble fragments) and pebbles which have undergone substantial collisions with dust and other pebbles. The Rosetta mission to the comet 67P/Churyumov-Gerasimenko and the New Horizons mission to Pluto will both provide valuable information about the structure of planetesimals in the Solar System. Our model predicts that 67P is a pebble-pile planetesimal consisting of primordial pebbles from the Solar Nebula, while the pebbles in the cloud which contracted to form Pluto must have been ground down substantially during the collapse.

To appear in: Astronomy & Astrophysics

For preprints, contact kalle@astro.lu.se
or on the web at http://arxiv.org/abs/1408.2535

Dumb-Bell-Shaped Equilibrium Figures for Fiducial Contact-Binary Asteroids and EKBOs
Pascal Descamps1

1 Institut de Mécanique Céleste et de Calcul des Éphémérides, Observatoire de Paris, UMR 8028 CNRS, 77 av. Denfert-Rochereau, 75014 Paris, France

In this work, we investigate the equilibrium figures of a dumb-bell-shaped sequence with which we are still not well acquainted. Studies have shown that these elongated and nonconvex figures may realistically replace the classic ``Roche binary approximation'' for modeling putative peanut-shaped or contact binary asteroids. The best-fit dumb-bell shapes, combined with the known rotational period of the objects, provide estimates of the bulk density of these objects. This new class of mathematical figures has been successfully tested on the observed light curves of three noteworthy small bodies: main-belt Asteroid 216 Kleopatra, Trojan Asteroid 624 Hektor and Edgeworth-Kuiper-belt object 2001 QG298. Using the direct observations of Kleopatra and Hektor obtained with high spatial resolution techniques and fitting the size of the dumb-bell-shaped solutions, we derived new physical characteristics in terms of equivalent radius, 62.5$\pm$5 km and 92$\pm$5 km, respectively, and bulk density, 4.4$\pm$0.4 g cm-3 and 2.43$\pm$0.35 g cm-3, respectively. In particular, the growing inadequacy of the radar shape model for interpreting any type of observations of Kleopatra (light curves, AO images, stellar occultations) in a satisfactory manner suggests that Kleopatra is more likely to be a dumb-bell-shaped object than a ``dog-bone.''

Published in: Icarus, 245, 64 (2015 January 1)

For preprints, contact descamps@imcce.fr
or on the web at http://arxiv.org/abs/1410.7962

The Intrinsic Neptune Trojan Orbit Distribution:
Implications for the Primordial Disk and Planet Migration
A.H. Parker1

1 Southwest Research Institute, 1050 Walnut St. Suite 300, Boulder CO, 80302, USA

The present-day orbit distribution of the Neptune Trojans is a powerful probe of the dynamical environment of the outer solar system during the late stages of planet migration. In this work, I conservatively debias the inclination, eccentricity, and libration amplitude distributions of the Neptune Trojans by reducing a priori unknown discovery and follow-up survey properties to nuisance parameters and using a likelihood-free Bayesian rejection sampler for parameter estimation. Using this survey-agnostic approach, I confirm that the Neptune Trojans are a dynamically excited population: at >95% confidence, the Neptune Trojans' inclination width must be $\sigma_i > 11^\circ$. For comparison and motivation purposes, I also model the Jupiter Trojan orbit distributions in the same basis and produce new estimates of their parameters (Jupiter Trojan $\sigma_i=14.4^\circ \pm 0.5^\circ$, $\sigma_{L11} = 11.8^\circ \pm 0.5^\circ$, and $\sigma_e = 0.061\pm
0.002$). The debiased inclination, libration amplitude, and eccentricity distributions of the Neptune Trojans are nominally very similar to those of the Jupiter Trojans. I use these new constraints to inform a suite of simulations of Neptune Trojan capture by an eccentric, rapidly-migrating Neptune from an initially dynamically-hot disk. These simulations demonstrate that if migration and eccentricity-damping timescales were short ( $\tau_a {\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$<$}}}10$ Myr, $\tau_e {\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$<$}}}1$ Myr), the disk that Neptune migrated into must have been pre-heated (prior to Neptune's appearance) to a width comparable to the Neptune Trojans' extant width to produce a captured population with an inclination distribution width consistent with that of the observed population.

To appear in: Icarus, 247, 112 (2015 February)

For preprints, contact alexharrisonparker@gmail.com
or on the web at http://arxiv.org/abs/1409.6735

Space Weathering and the Color-Color Diagram of Plutinos and Jupiter Trojans
M.D. Melita1, Z. Kanuchová2, R. Brunetto3, and G. Strazzulla4

1 Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina
2 Astronomical Institute of Slovak Academy of Sciences, SK-059 60 T. Lomnica, Slovakia
3 Institut d'Astrophysique Spatiale, CNRS, UMR-8617, Université Paris-Sud, bâtiment 121, F-91405 Orsay Cedex, France
4 INAF-Osservatorio Astrofisico di Catania, Via S. Sofia 78, I-95123 Catania, Italy

The Jupiter Trojan asteroids and the Plutinos are two peculiar populations. They are dynamically resonant, therefore with heliocentric distances relatively bounded for long timescales, as a fairly general rule. As a consequence, some correlation with the surface color properties of their respective members is expected. Indeed, there are apparent differences in the B-V vs. V-R color-color diagram of the two populations. Using a simple model based on the surface color due to the contribution of two components, one pristine and one altered, we find as plausible that the difference is due to the interplay of space weathering by energetic cosmic-radiation and collisional effects.

To appear in: Icarus

For preprints, contact melita@iafe.uba.ar

Infrared Spectra and Optical Constants of Astronomical Ices:
II. Ethane and Ethylene
R.L. Hudson1, P.A. Gerakines1, and M.H. Moore1

1Astrochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Infrared spectroscopic observations have established the presence of hydrocarbon ices on Pluto and other TNOs, but the abundances of such molecules cannot be deduced without accurate optical constants (n, k) and reference spectra. In this paper we present our recent measurements of near- and mid-infrared optical constants for ethane (C2H6) and ethylene (C2H4) in multiple ice phases and at multiple temperatures. As in our recent work on acetylene (C2H2), we also report new measurements of the index of refraction of each ice at 670$\,$nm. Comparisons are made to earlier work where possible, and electronic versions of our new results are made available.

Published in: Icarus 243, 148 (2014 November 15)

For preprints, contact reggie.hudson@nasa.gov
Download data and preprints on the web at http://science.gsfc.nasa.gov/691/cosmicice


Plaskett Research Associate
National Science Infrastructure Portfolio (NSI)
NRC Herzberg Astronomy and Astrophysics
Dominion Astrophysical Observatory (DAO)
5071 West Saanich Road
Victoria, BC V9E 2E7
Fax: 709-772-4541
Telephone: 709-772-5012

E-mail enquiries: NRC.ExtHiring-EmbaucheExt.CNRC@nrc-cnrc.gc.ca
Related URL: http://www.nrc-cnrc.gc.ca/eng/careers/index.html

Desired starting date: 1 September 2015

The National Research Council (NRC) is pleased to announce the 2014 competition for two Plaskett Postdoctoral Research Associate (RA) positions tenable at DAO in Victoria, BC, Canada. The successful candidates will be outstanding recent doctoral graduates in astrophysics who are highly motivated to exploit facilities administered by NRC for Canadian astronomers and contribute to projects led by NRC Herzberg staff members. We are particularly seeking candidates who will contribute to one or more of the following research areas:

The successful candidates are expected to work independently and perform original research in collaboration with NRC Herzberg staff members associated with projects most relevant to his or her area of expertise. Additionally, candidates are expected to keep an active engagement with the community to advance NRC Herzberg's mandate. In particular, NRC Herzberg is involved in several ground- and space-based telescopes of the present (e.g., ALMA, CFHT, Gemini, JCMT) and future (e.g., JWST, CASTOR, Astrosat, WFIRST, Astro-H, SKA, MSE, TMT), as well as in scientific data preservation, distribution, and analysis techniques. The Canadian Astronomy Data Centre, home to the Canadian Virtual Observatory, the CANFAR cloud computing network, and data archives including, e.g., CFHT, Canadian Galactic Plane Survey, Gemini, HST, and JCMT, are also located at NRC Herzberg. Candidates with interests in developing studies of next-generation instrumentation and facilities will be given highest consideration.

The successful candidates will share with other Plaskett RAs the organization of the weekly seminar series that runs September-April, and have access via Canada's TAC process to the astronomical facilities operated by NRC, as well as other facilities with open competition. The positions will constitute initial appointments of two years, which may be extended for one further year (subject to performance and availability of funds). In addition to highly competitive benefits and salary, Plaskett RAs will receive support for observing and conference travel, page charges, and access to professionally managed computers.

Applicants must have acquired their Ph.D. within the last five years or expect to obtain their degree before taking up the position. Applications should be made by 12 December 2014 via the process described at the URL provided.

NRC is an equal opportunity employer.

Newsletter Information

The Distant EKOs Newsletter is dedicated to provide researchers with easy and rapid access to current work regarding the Kuiper belt (observational and theoretical studies), directly related objects (e.g., Pluto, Centaurs), and other areas of study when explicitly applied to the Kuiper belt.

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Distant EKOs is not a refereed publication, but is a tool for furthering communication among people interested in Kuiper belt research. Publication or listing of an article in the Newsletter or the web page does not constitute an endorsement of the article's results or imply validity of its contents. When referencing an article, please reference the original source; Distant EKOs is not a substitute for peer-reviewed journals.

Moving ... ??

If you move or your e-mail address changes, please send the editor your new address. If the Newsletter bounces back from an address for three consecutive issues, the address will be deleted from the mailing list. All address changes, submissions, and other correspondence should be sent to:

Joel Parker 2014-11-12