Distant EKOs, Issue #103  (February 2016)


News & Announcements
Abstracts of 4 Accepted Papers
Abstracts of 1 Submitted Paper
Abstracts of 1 Other Paper of Interest
Newsletter Information


Having completed its successful flyby of the Pluto system, the New Horizons spacecraft is on a trajectory to encounter Kuiper Belt Object 2014 MU69. Pending NASA approval for an extended mission, New Horizons will also take advantage of being an observing platform in the outer solar system to observe a select number of other KBOs having favorable geometries for resolved or high signal-to-noise measurements.

Earth-based observations can support these pending New Horizons measurements through calibrated photometry at low phase angle (Earth), which will be complementary to the higher phase angle data from the spacecraft. In particular for objects having the potential for resolved imaging from the spacecraft, knowledge of the rotational phase at the time of the New Horizons observations can help constrain the overall shape of these distant objects.

An Earth-based campaign website in support of the pending science from the New Horizons extended mission is under construction http://www.boulder.swri.edu/nh-support-obs/kbo. Available there now is a table listing the pending targets having the highest priority for supporting observations. Register on that page to indicate interest.

There were 4 new TNO discoveries announced since the previous issue of Distant EKOs:

2012 HE85, 2015 BZ517, 2015 QT11, 2015 SP21

and 4 new Centaur/SDO discoveries:

2015 KH162, 2015 PK312, 2015 SO21, 2016 AE193

Reclassified objects:

2016 AE193 (SDO $\rightarrow$ Centaur)
2001 OO108 (TNO $\rightarrow$ SDO)
2002 PR170 (TNO $\rightarrow$ SDO)

Objects recently assigned numbers:

2013 XZ8 = (459865)
2014 AT28 = (459870)
2014 ON6 = (459971)
1999 OM4 = (455171)
2001 FE193 = (455206)
2001 KT76 = (455209)
2003 UZ413 = (455502)
2007 TH422 = (456826)

Objects recently assigned names:

2012 BX85 = Praamzius

Current number of TNOs: 1453 (including Pluto)
Current number of Centaurs/SDOs: 489
Current number of Neptune Trojans: 12

Out of a total of 1954 objects:
   659 have measurements from only one opposition
     639 of those have had no measurements for more than a year
       328 of those have arcs shorter than 10 days
(for more details, see: http://www.boulder.swri.edu/ekonews/objects/recov_stats.jpg)


The Long-wavelength Thermal Emission of the Pluto-Charon System from Herschel Observations. Evidence for Emissivity Effects
E. Lellouch1, P. Santos-Sanz2, S. Fornasier1, T. Lim3, J. Stansberry4, E. Vilenius5,6, Cs. Kiss7, T. Müller6, G. Marton7, S. Protopapa8, P. Panuzzo9, and R. Moreno1

1LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
2 Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008-Granada, Spain
3 European Space Astronomy Centre (ESAC), P.O. Box 78, E-28691 Villanueva de la Cañada, Madrid, Spain
4 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 USA
5 Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
6 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
7 Konkoly Observatory of the Hungarian Academy of Sciences, H-1121 Budapest, Konkoly Thege Miklós út 15-17, Hungary
8 Department of Astronomy, University of Maryland, College Park, MD 20742, USA
9 GEPI, Observatoire de Paris, PSL Research University, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France

Thermal observations of the Pluto-Charon system acquired by the Herschel Space Observatory in February 2012 are presented. They consist of photometric measurements with the PACS and SPIRE instruments (nine visits to the Pluto system each), covering six wavelengths from 70 to 500 $\mu$m altogether. The thermal light curve of Pluto-Charon is observed in all filters, albeit more marginally at 160 and especially 500 $\mu$m. Putting these data into the context of older ISO, Spitzer and ground-based observations indicates that the brightness temperature (TB) of the system (rescaled to a common heliocentric distance) drastically decreases with increasing wavelength, from $\sim$53 K at 20 $\mu$m to $\sim$35 K at 500 $\mu$m, and perhaps even less at longer wavelengths. Considering a variety of diurnal and/or seasonal thermophysical models, we show that TB values of 35 K are lower than any expected temperature for the dayside surface or subsurface of Pluto and Charon, implying a low surface emissivity. Based on multiterrain modeling, we infer a spectral emissivity that decreases steadily from 1 at 20-25 $\mu$m to $\sim$0.7 at 500 $\mu$m. This kind of behavior is usually not observed in asteroids (when proper allowance is made for subsurface sounding), but is found in several icy surfaces of the solar system. We tentatively identify that a combination of a strong dielectric constant and a considerable surface material transparency (typical penetration depth $\sim$1 cm) is responsible for the effect. Our results have implications for the interpretation of the temperature measurements by REX/New Horizons at 4.2 cm wavelength.

To appear in: Astronomy & Astrophysics

Preprints available on the web at http://arxiv.org/abs/1601.05606

Observational Constraint on Pluto's Atmospheric CO with ASTE
T. Iino1, Y. Hirahara2, T. Hidemori3, T. Tsukagoshi4, T. Nakajima3, S. Nakamoto2, and C. Kato3

1 Nature and Science Museum, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan
2 Graduate School of Environment, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
3 Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan
4 School of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan

To confirm the previous observational results of Pluto's atmospheric CO in the J = 2-1 rotational transition, we conducted a new observation of CO (J = 3-2) in Pluto's atmosphere in 2014 August with the Atacama Submillimeter Telescope Experiment 10 m single-dish telescope. In contrast to the previous observational result obtained with the James Clerk Maxwell Telescope in 2009 and 2010 by using the J = 2-1 transition, no emission structure was observed near the rest frequency in our attempt. Possible explanations for the nondetection result of the J = 3-2 transition are discussed.

Published in: Publications of the Astronomical Society of Japan, 68, L1

Dynamical and Collisional Evolution of Kuiper Belt Binaries
Adrián Brunini1 and Macarena Zanardi2

1 Unidad Académica Caleta Olivia, Universidad Nacional de la Patagonia Austral. Ruta 3 Acceso Norte, Santa Cruz (9011) Argentina
2 Grupo de Ciencias Planetarias, Facultad de Ciencias Astronómicas y Geofísicas. Paseo del Bosque s/n (1900) La Plata, Argentina

We present numerical simulations of the evolution of synthetic Trans-Neptunian Binaries (TNBs) under the influence of the solar perturbation, tidal friction, and collisions with the population of Classical Kuiper Belt Object (KBOs).

We show that these effects, acting together, have strongly sculpted the primordial population of TNBs. If the population of Classical KBOs have a power law size distribution as the ones that are inferred from recent observational surveys (Petit et al. 2011, Adams et al. 2014, Fraser at al. 2014), the fraction of surviving binaries at present would be $\sim$70% of the primordial population. The orbits of the surviving synthetic systems match reasonably well the observed sample.

The collisional process excites the mutual orbital eccentricity of the binaries, acting against the effect of tides. Therefore only $\sim$10% of the objects reach total orbital circularization ( $e \leq 10^{-4}$). In addition, our results predict that the population of contact binaries in the Trans Neptunian region should be small.

Ultra wide binaries are naturally obtained by the combined action of Kozai Cycles and Tidal Friction (KCTF) and collisional evolution, being the number and orbital distribution of them very similar to the ones of the observed population.

Published in: Monthly Notices of the Royal Astronomical Society, 455, 4487

(2016 February 1)

For preprints, contact abrunini@yahoo.com.ar

On the Mass and Origin of Chariklo's Rings
Margaret Pan1,2 and Yanqin Wu1

1 University of Toronto, Toronto, Ontario, Canada
2 MIT, Cambridge, Massachusetts, USA

Observations in 2013 and 2014 of the Centaur 10199 Chariklo and its ring system consistently indicated that the radial width of the inner, more massive ring varies with longitude. That strongly suggests that this ring has a finite eccentricity despite the fast differential precession that Chariklo's large quadrupole moment should induce. If the inferred apse alignment is maintained by the ring's self-gravity, as it is for the Uranian rings, we estimate a ring mass of a few times 1016 g and a typical particle size of a few meters. These imply a short collisional spreading time of $\sim10^5$ years, somewhat shorter than the typical Centaur dynamical lifetime of a few Myrs and much shorter than the age of the solar system. In light of this time constraint, we evaluate previously suggested ring formation pathways including collisional ejection and satellite disruption. We also investigate in detail a contrasting formation mechanism, the lofting of dust particles off Chariklo's surface into orbit via outflows of sublimating CO and/or N2 triggered after Chariklo was scattered inward by giant planets. This latter scenario predicts that rings should be common among 100-km class Centaurs but rare among Kuiper belt objects and smaller Centaurs. It also predicts that Centaurs should show seasonal variations in cometary activity with activity maxima occurring shortly after equinox.

To appear in: The Astrophysical Journal

For preprints, contact http://pan at astro dot utoronto dot ca
or on the web at http://arxiv.org/abs/1602.01769


Pluto's Atmosphere from the 29 June 2015 Ground-based Stellar Occultation at the Time of The New Horizons Flyby
B. Sicardy1, J. Talbot2, E. Meza1, et al.

1 LESIA/Observatoire de Paris, PSL, CNRS UMR 8109, University Pierre et Marie Curie, University Paris-Diderot, 5 place Jules Janssen, F-92195 Meudon Cédex, France
2 Occultation Section of the Royal Astronomical Society of New Zealand (RASNZ)

We present results from a multi-chord Pluto stellar occultation observed on 29 June 2015 from New Zealand and Australia. This occurred only two weeks before the NASA New Horizons flyby of the Pluto system and serves as a useful comparison between ground-based and space results. We find that Pluto's atmosphere is still expanding, with a significant pressure increase of 5$\pm$2% since 2013 and a factor of almost three since 1988. This trend rules out, as of today, an atmospheric collapse associated with Pluto's recession from the Sun. A central flash, a rare occurrence, was observed from several sites in New Zealand. The flash shape and amplitude are compatible with a spherical and transparent atmospheric layer of roughly 3 km in thickness whose base lies at about 4 km above Pluto's surface, and where an average thermal gradient of about 5 K km-1 prevails. We discuss the possibility that small departures between the observed and modeled flash are caused by local topographic features (mountains) along Pluto's limb that block the stellar light. Finally, using two possible temperature profiles, and extrapolating our pressure profile from our deepest accessible level down to the surface, we estimate a range of 12.4-13.2 $\mu$bar for the surface pressure.

Submitted to: Astrophysical Journal Letters

For preprints, contact bruno.sicardy@obspm.fr
or on the web at http://arxiv.org/abs/1601.05672


Comet Formation in Collapsing Pebble Clouds. What Cometary Bulk Density Implies for the Cloud Mass and Dust-to-ice Ratio
S. Lorek1, B. Gundlach2, P. Lacerda1, and J. Blum2

1 Max-Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
2 Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany

Comets are remnants of the icy planetesimals that formed beyond the ice line in the Solar Nebula. Growing from $\mu$m-sized dust and ice particles to km-sized objects is, however, difficult because of growth barriers and time scale constraints. The gravitational collapse of pebble clouds that formed through the streaming instability may provide a suitable mechanism for comet formation. We study the collisional compression of silica, ice, and silica/ice-mixed pebbles during gravitational collapse of pebble clouds. Using the initial volume-filling factor and the dust-to-ice ratio of the pebbles as free parameters, we constrain the dust-to-ice mass ratio of the formed comet and the resulting volume-filling factor of the pebbles, depending on the cloud mass. We use the representative particle approach, which is a Monte Carlo method, to follow cloud collapse and collisional evolution of an ensemble of ice, silica, and silica/ice-mixed pebbles. Therefore, we developed a collision model which takes the various collision properties of dust and ice into account. We study pebbles with a compact size of 1 cm and vary the initial volume-filling factors, $\phi_0$, ranging from 0.001 to 0.4. We consider mixed pebbles as having dust-to-ice ratios between 0.5 and 10. We investigate four typical cloud masses, M, between 2.6 x 1014 g (very low) and 2.6 x 1023 g (high). Except for the very low-mass cloud ( M=2.6 x 1014 g), silica pebbles are always compressed during the collapse and attain volume-filling factors in the range from $\langle\phi\rangle_V\approx0.22$ to 0.43, regardless of $\phi_0$. Ice pebbles experience no significant compression in very low-mass clouds. They are compressed to values in the range $\langle\phi\rangle_V\approx0.11$ to 0.17 in low- and intermediate-mass clouds ( M=2.6 x 1017 - 2.6 x 1020 g); in high-mass clouds ( M=2.6 x 1023 g), ice pebbles end up with $\langle\phi\rangle_V\approx0.23$. Mixed pebbles obtain filling factors in between the values for pure ice and pure silica. We find that the observed cometary density of $\sim$0.5 g cm-3 can only be explained by either intermediate- or high-mass clouds, regardless of $\phi_0$, and also by either very low- or low-mass clouds for initially compact pebbles. In any case, the dust-to-ice ratio must be in the range of between $3{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$<$}}}\xi{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$<$}}}9$ to match the observed bulk properties of comet nuclei.

To appear in: Astronomy & Astrophysics

For preprints, contact lorek@mps.mpg.de
or on the web at http://arxiv.org/abs/1601.05726

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.

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Joel Parker 2016-02-29