Distant EKOs, Issue #93  (2014 June)


News & Announcements
Abstracts of 8 Accepted Papers
Newsletter Information


There were 8 new TNO discoveries announced since the previous issue of Distant EKOs:

2013 FA28, 2013 FB28, 2013 FC28, 2013 FD28, 2013 JV63, 2013 JW63, 2014 FP43, 2014 FX61

and 6 new Centaur/SDO discoveries:

2013 JU63, 2014 FU61, 2014 FB62, 2014 KL84, 2014 LR14, 2014 LJ9

Reclassified objects:

2014 FB62 (Centaur $\rightarrow$ TNO)

Objects recently assigned numbers:

2012 GN12 = (395699)

Current number of TNOs: 1272 (including Pluto)
Current number of Centaurs/SDOs: 398
Current number of Neptune Trojans: 9

Out of a total of 1679 objects:
   648 have measurements from only one opposition
     631 of those have had no measurements for more than a year
       327 of those have arcs shorter than 10 days
(for more details, see: http://www.boulder.swri.edu/ekonews/objects/recov_stats.jpg)


De-biased Populations of Kuiper Belt Objects from the Deep Ecliptic Survey
E.R. Adams1, A.A.S. Gulbis2,3, J.L. Elliot3,4, S.D. Benecchi1,5, M.W. Buie6, D.E. Trilling7, and L.H. Wasserman8

1 Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA
2 The Southern African Large Telescope and South African Astronomical Observatory, Cape Town, South Africa, 7935
3 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA, 02139, USA
4 Deceased
5 Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA
6 Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
7 Department of Physics & Astronomy, Northern Arizona University, S San Francisco St, Flagstaff, AZ 86011, USA
8 Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA

The Deep Ecliptic Survey (DES) was a survey project that discovered hundreds of Kuiper Belt objects from 1998-2005. Extensive follow-up observations of these bodies has yielded 304 objects with well-determined orbits and dynamical classifications into one of several categories: Classical, Scattered, Centaur, or 16 mean-motion resonances with Neptune. The DES search fields are well documented, enabling us to calculate the probability on each frame of detecting an object with its particular orbital parameters and absolute magnitude at a randomized point in its orbit. The detection probabilities range from a maximum of 0.32 for the 3:2 resonant object 2002 GF32 to a minimum of 1.5 x 10-7 for the faint Scattered object 2001 FU185. By grouping individual objects together by dynamical classes, we can estimate the distributions of four parameters that define each class: semi-major axis, eccentricity, inclination, and object size. The orbital element distributions (a, e, and i) were fit to the largest three classes (Classical, 3:2, and Scattered) using a maximum likelihood fit. Using the absolute magnitude (H-magnitude) as a proxy for the object size, we fit a power law to the number of objects vs. H magnitude for 8 classes with at least 5 detected members (246 objects). The Classical objects are best fit with a power-law slope of $\alpha=1.02\pm0.01$ (observed from $5 \le H \le 7.2$). Six other dynamical classes (Scattered plus 5 resonances) have consistent magnitude distributions slopes with the Classicals, provided that the absolute number of objects is scaled. Scattered objects are somewhat more numerous than Classical objects, while there are only a quarter as many 3:2 objects as Classicals. The exception to the power law relation is the Centaurs, which are non-resonant objects with perihelia closer than Neptune and therefore brighter and detectable at smaller sizes. Centaurs were observed from 7.5<H<11, and that population is best fit by a power law with $\alpha = 0.42\pm0.02$. This is consistent with a knee in the H-distribution around H=7.2 as reported elsewhere (Bernstein et al. 2004, Fraser et al. 2014). Based on the Classical-derived magnitude distribution, the total number of objects ($H\le7$) in each class are: Classical ($2100\pm300$ objects), Scattered ($2800\pm 400$), 3:2 ($570\pm 80$), 2:1 ($400\pm 50$), 5:2 ($270\pm 40$), 7:4 ($69\pm 9$), 5:3 ($60\pm 8$). The independent estimate for the number of Centaurs in the same H range is $13\pm 5$. If instead all objects are divided by inclination into ``Hot'' and ``Cold'' populations, following Fraser et al. (2014), we find that $\alpha_{Hot} =0.90\pm0.02$, while $\alpha_{Cold} = 1.32\pm0.02$, in good agreement with that work.

To appear in: The Astronomical Journal

For preprints, contact adams@psi.edu
or on the web at http://arxiv.org/abs/1311.3250

New Horizons: Long-range Kuiper Belt Targets Observed by the Hubble Space Telescope
S.D. Benecchi1,2, K.S. Noll3, H.A. Weaver4, J.R. Spencer5, S.A. Stern5, M.W. Buie5, and A.H. Parker6

1 Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA
2 Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
3 NASA Goddard Space Fight Center, 8800 Greenbelt Rd. Code 693, Greenbelt, MD 20771, USA
4 Space Department, Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
5 Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA
6 Department of Astronomy, University of California at Berkeley, B-20 Hearst Field Annex # 3411, Berkeley, CA 94720, USA

We report on Hubble Space Telescope (HST) observations of three Kuiper Belt Objects (KBOs), discovered in our dedicated ground-based search campaign, that are candidates for long-range observations from the New Horizons spacecraft: 2011 JY31, 2011 HZ102, and 2013 LU35. Astrometry with HST enables both current and future critical accuracy improvements for orbit precision, required for possible New Horizons observations, beyond what can be obtained from the ground. Photometric colors of all three objects are red, typical of the Cold Classical dynamical population within which they reside; they are also the faintest KBOs to have had their colors measured. None are observed to be binary with HST above separations of $\sim0.02$ arcsec ($\sim700$ km at 44 AU) and $\Delta m \leq 0.5$.

Published in: Icarus

For preprints, contact susank@psi.edu
or on the web at http://arxiv.org/abs/1405.7181
and at http://dx.doi.org/10.1016/j.icarus.2014.04.014

A Study of the High-inclination Population in the Kuiper Belt - II. The Twotinos
J. Li1, L.-Y. Zhou1, and Y.-S. Sun1

1 School of Astronomy and Space Science & Key Laboratory of Modern Astronomy and Astrophysics in Ministry of Education, Nanjing University, Nanjing 210093, PR China

As the second part of our study, in this paper we proceed to explore the dynamics of the high-inclination Twotinos in the 1:2 Neptune mean motion resonance (NMMR). Depending on the inclination i, we show the existence of two critical eccentricities ea(i) and ec(i), which are lower limits of the eccentricity e for the resonant angle $\sigma$ to exhibit libration and asymmetric libration, respectively. Accordingly, we have determined the libration centres $\sigma_0$ for inclined orbits, which are strongly dependent on i. With initial $\sigma=\sigma_0$ on a fine grid of (e, i), the stability of orbits in the 1:2 NMMR is probed by 4-Gyr integrations. It is shown that symmetric librators are totally unstable for $i\ge30^{\circ}$; while stable asymmetric librators exist for i up to $90^{\circ}$.

We further investigate the 1:2 NMMR capture and retention of planetesimals with initial inclinations $i_0\le90^{\circ}$ in the planet migration model using a long time-scale of 2 x 107 yr. We find that: (1) the capture efficiency of the 1:2 NMMR decreases drastically with the increase of i0, and it goes to 0 when $i_0\ge60^{\circ}$; (2) the probability of discovering Twotinos with $i>25^{\circ}$, beyond observed values, is roughly estimated to be $\le0.1$ per cent; (3) more particles are captured into the leading rather than the trailing asymmetric resonance for $i_0\le10^{\circ}$, but this number difference appears to be the opposite at $i_0=20^{\circ}$ and is continuously varying for even larger i0; (4) captured Twotinos residing in the trailing resonance or having $i>15^{\circ}$ are practically outside the Kozai mechanism, like currently observed samples.

To appear in: Monthly Notices of the Royal Astronomical Society

Preprints available on the web at http://arxiv.org/abs/1402.5924

Extreme Trans-Neptunian Objects and the Kozai Mechanism: Signaling the Presence of Trans-Plutonian Planets?
C. de la Fuente Marcos1 and R. de la Fuente Marcos1

1 Universidad Complutense de Madrid, Ciudad Universitaria, E-28040, Madrid, Spain

The existence of an outer planet beyond Pluto has been a matter of debate for decades and the recent discovery of 2012 VP113 has just revived the interest for this controversial topic. This Sedna-like object has the most distant perihelion of any known minor planet and the value of its argument of perihelion is close to 0$^{\circ}$. This property appears to be shared by almost all known asteroids with semimajor axis greater than 150 au and perihelion greater than 30 au (the extreme trans-Neptunian objects or ETNOs), and this fact has been interpreted as evidence for the existence of a super-Earth at 250 au. In this scenario, a population of stable asteroids may be shepherded by a distant, undiscovered planet larger than the Earth that keeps the value of their argument of perihelion librating around 0$^{\circ}$ as a result of the Kozai mechanism. Here, we study the visibility of these ETNOs and confirm that the observed excess of objects reaching perihelion near the ascending node cannot be explained in terms of any observational biases. This excess must be a true feature of this population and its possible origin is explored in the framework of the Kozai effect. The analysis of several possible scenarios strongly suggest that at least two trans-Plutonian planets must exist.

To appear in: Monthly Notices of the Royal Astronomical Society, Letters

For preprints, contact nbplanet@fis.ucm.es
or on the web at http://arxiv.org/abs/1406.0715

Large Retrograde Centaurs: Visitors from the Oort Cloud?
C. de la Fuente Marcos1 and R. de la Fuente Marcos1

1 Universidad Complutense de Madrid, Ciudad Universitaria, E-28040, Madrid, Spain

Among all the asteroid dynamical groups, Centaurs have the highest fraction of objects moving in retrograde orbits. The distribution in absolute magnitude, H, of known retrograde Centaurs with semi-major axes in the range 6-34 AU exhibits a remarkable trend: 10% have H < 10 mag, the rest have H > 12 mag. The largest objects, namely (342842) 2008 YB3, 2011 MM4 and 2013 LU28, move in almost polar, very eccentric paths; their nodal points are currently located near perihelion and aphelion. In the group of retrograde Centaurs, they are obvious outliers both in terms of dynamics and size. Here, we show that these objects are also trapped in retrograde resonances that make them unstable. Asteroid 2013 LU28, the largest, is a candidate transient co-orbital to Uranus and it may be a recent visitor from the trans-Neptunian region. Asteroids 342842 and 2011 MM4 are temporarily submitted to various high-order retrograde resonances with the Jovian planets but 342842 may be ejected towards the trans-Neptunian region within the next few hundred kyr. Asteroid 2011 MM4 is far more stable. Our analysis shows that the large retrograde Centaurs form an heterogeneous group that may include objects from various sources. Asteroid 2011 MM4 could be a visitor from the Oort cloud but an origin in a relatively stable closer reservoir cannot be ruled out. Minor bodies like 2011 MM4 may represent the remnants of the primordial planetesimals and signal the size threshold for catastrophic collisions in the early Solar System.

To appear in: Astrophysics and Space Science

For preprints, contact nbplanet@fis.ucm.es
or on the web at http://arxiv.org/abs/1406.1450

On the State of Methane and Nitrogen Ice on Pluto and Triton: Implications of the Binary Phase Diagram
L.M. Trafton1

1 University of Texas at Austin, Department of Astronomy, 2515 Speedway, C1400, Austin, TX 78712-1205, USA

Compositional analyses of Pluto's surface ice in the literature typically include large areas on the body where CH4 and other volatiles are segregated in the pure form from the solid solution N2:CH4 in which CH4 is diluted. However, the existence of continent-size areas of pure CH4 are in conflict with both of the alternative models that successfully explain the enhancement of CH4 in Pluto's atmosphere, the Detailed Balancing thermal equilibrium model and the Hot Methane Patch model. Pluto's spectrum includes an apparently unshifted CH4 component while Triton's does not, and 93% of the concentration range of the binary phase diagram at 38 K shows that these species exist as a mixture of two saturated solid solution phases. Recognizing this, we propose that both of these saturated phases are present on Pluto and the CH4-rich phase of the mixture, CH4:N2, is the source of the relatively unshifted CH4 spectrum attributed to pure CH4. We also propose that CH4 is less abundant in Triton's ice to the point where either the ice is not saturated or the saturated CH4:N2 phase has not been detected. In this scenario, the partial vapor pressures do not change when the relative proportions of these saturated phases are varied in the mixture. Thus, the partial vapor pressures are independent of N2-CH4 concentrations if both saturated phases are present. Accordingly, the longitudinal and seasonal variations of CH4 and N2 features in Pluto's spectrum would be attributed to spatial variations in the relative proportions of these species. This may occur during volatile transport in the sublimation wind through extensive influences. The lower, unsaturated, values of the mole fraction of CH4 in the ice reported by Owen et al. (1993) and Cruikshank et al. (1998), and by Doute et al. (1999) based on a compositional analysis of Pluto's surface, were not obtained using optical constants for components consistent with the constraints of the phase diagram.

To appear in: Icarus

For preprints, contact lmt@astro.as.utexas.edu

Gas Transfer in the Pluto-Charon System: A Charon Atmosphere
O.J. Tucker1, R.E. Johnson2,3, and L.A. Young4

1 Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA
2 Department of Engineering Physics, University of Virginia, Charlottesville, VA 22904, USA
3 Department of Physics, New York University, NY, NY 10003, USA
4 Southwest Research Institute, Boulder, CO 80302, USA

Recent hybrid fluid/molecular kinetic models demonstrated that Pluto’s upper atmosphere is warmer and more extended than previously thought (Erwin et al., 2013; Tucker et al. 2012). Here we examine the effect of Charon on the molecular escape rate from Pluto’s extended atmosphere, simulate the spatial distribution of N2 in this binary system, and describe the resulting accumulation of N2 on Charon. These Monte Carlo simulations are carried out for approximate solar medium conditions at $\sim$33 AU. Including Charon’s gravity and orbital motion,the atmosphere on the Pluto’s Charon-facing hemisphere is more strongly bound to the system and is more extended than the atmosphere on Pluto’s anti-Charon hemisphere. Accounting for Charon’s gravity the net escape from the system is reduced by $\sim$5%. Most of the loss is direct from Pluto’s exobase region with $\sim$1-2% due to scattering by Charon. About $\sim$0.2% of the flux from Pluto’s exobase impinges on Charon: $\sim 5.7 \times 10^{25}$ N2/s at nominal solar medium conditions. This is a source of condensed N2 for Charon’s night side and forms a tenuous atmosphere. For the approximate range of surface temperatures, the residence time of N2 on the surface can range from a fraction of a second to 10’s of years with the near surface line of sight column densities varying from $\sim 3 \times 10^{18}$ N2/m2 up to > 6 x 1019 N2/m2. Such an atmosphere could be detectable during the solar occultation that will occur during the New Horizon encounter providing a measure of the transfer of gas between bodies in this binary system.

To appear in: Icarus

For preprints, contact ojtucker@umich.edu
or on the web at http://dx.doi.org/10.1016/j.icarus.2014.05.002

The Effect of Rayleigh-Taylor Instabilities on the Thickness of Undifferentiated Crust on Kuiper Belt Objects
M.E. Rubin1, S.J. Desch1, and M. Neveu1

1 School of Earth and Space Exploration, Arizona State University, P.O. Box 871404, Tempe, AZ 85287-1404, USA

Previous calculations of the internal structure and thermal evolution of Kuiper Belt Objects (KBOs) by Desch et al. (Desch, S.J., Cook, J.C., Doggett, T.C., Porter, S.B. [2009]. Icarus 202, 694-714) have predicted that KBOs should only partially differentiate, with rock and ice separating into a rocky core and icy mantle, below an undifferentiated crust of ice and rock. This crust is thermally insulating and enhances the ability of subsurface liquid to persist within KBOs. A dense rock/ice layer resting on an icy mantle is gravitationally unstable and prone to Rayleigh-Taylor (RT) instabilities, and may potentially overturn. Here we calculate the ability of RT instabilities to act in KBOs, and determine the thickness of undifferentiated crusts. We have used previously calculated growth rates of the RT instability to determine the critical viscosity of ice needed for the RT instability to operate. We calculate the viscosity of ice at the cold temperatures and long timescales relevant to KBOs. We find that crustal overturn is only possible where the temperature exceeds about 150 K, and that RT instabilities cannot act on geological timescales within about 60 km of the surfaces of a KBO like Charon. Although this crustal thickness is less than the 85 km previously calculated by Desch et al. (Desch, S.J., Cook, J.C., Doggett, T.C., Porter, S.B. [2009]. Icarus 202, 694-714), it is still significant, representing $\sim$25% of the mass of the KBO. We conclude that while RT instabilities may act in KBOs, they do not completely overturn their crusts. We calculate that Saturn’s moon Rhea should only partially differentiate, resulting in a moment of inertia $C/MR^2 \approx 0.38$.

Published in: Icarus, 236, 122 (2014 July)

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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Joel Parker 2014-07-03